The first two books are direct parodies of Sword and Sorcery fantasy. They featured a lot of elements that were quietly dropped in the later books, which are parodies of just about everything.
The shift in human naming conventions is particularly marked, with fantasy-parodic names like Zlorf and Gorphal giving way to conventional names like Fred or Sybil. Equal Rites saw the first ordinary surnames (the Smiths), and Mort (for Mortimer) was the series' first protagonist with a real-world given name. The more fantastic names are reserved for non-human species in later books.
The Patrician who appears in the first book is unnamed and bears absolutely no resemblance to our favorite Magnificent Bastard Lord Vetinari, to the point where Pterry had to confirm that it was him. In Sourcery he is completely impotent; he's casually transformed into a small lizard by Coin, and spends most of the book that way.
Pterry is also more likely to Call a Rabbit a "Smeerp" in the early novels; Mort's father apparently farms "tharga beasts", and since (unlike vermine or republican bees in later books) nothing amusing is said about them to differ them from Roundworld animals, they might as well be oxen.
Much is made of Nobby Nobbs being a clothes horse in Guards! Guards! and having a wardrobe of the latest fashions that he wears off duty. By Feet of Clay, he has no idea what to wear to a posh party until Fred Colon tells him he can just wear his uniform.
Death actively, and with little reason, kills a man in the first book by stopping his heart. This goes directly against his personality in later books, as he knows fully well that people are supposed to die on their own time. Also, he rather likes people.
The city of Quirm was originally generically 'foreign' in contrast to the British Ankh-Morpork; in fact before Brindisi became established as the Disc version of Italy, Quirm was sometimes a stand in for things Italian (mention is made of a Pisan-style 'Collapsed Tower' and the Leonardo da Vinci analog is Leonard da Quirm). In later books it is very firmly fantasy counterpart France.
The first fairly major dwarf character was Hwel from Wyrd Sisters, and much was made of him being a great playwright, since most dwarfs apparently cannot even read. However, practically all other books portray dwarves of not only being literate, but deeply respecting the written word to the point where it almost seems like a religion. (The dwarves are deeply awed by the concept of a “Blackboard Monitor”, somebody trusted enough to erase words.) Although Hwel is still an unusual character, because the writing most dwarves do seems to be for legal, religious, or professional purposes (treaties/contracts, signage, and letters) rather than creative writing. So Hwel is still Square Race, Round Class, even if dwarves can now read.
Guards! Guards! has the adopted dwarf, Carrot, make reference to female dwarfs. Additionally a minor character in Soul Music is a openly female dwarf, and nobody considers this unusual. Starting with Feet of Clay, openly female dwarves are an anomaly, with most forced to hide their true gender by society.
On Basilisk Station, the first Honor Harrington novel, has a bit of this. The extremely short-ranged Grav Lance that is one of the central plot elements of the book is gone from later installments, as weapon ranges extend from tens to hundreds of thousands to hundreds of millions of kilometers, and while Admiral Dame Sonja Hemphill remains quite enamored of her own genius in her later appearancesnote and not undeservedly, given that her technological breakthroughs include faster-than-light communication, multi-drive missiles, and FTL missile control, plus a host of others, she is not (unlike in the first book) inclined to take out her frustrations over the failures of systems she developed on the officers trying to employ them in the field.
The first novel, Redwall, features a number of references indicating that the animals live in a world where humans also exist, such as a horse cart, a church, taverns, ports, and a direct mention of Portugal. Also, one of the characters was a beaver. In later books, author Brian Jacques made it clear that only animals existed in the Redwall universe, and only animals native to the British Isles, so there were no future appearances of any more beavers (though beavers WERE native to Britain at one time, but they were killed off due to overhunting). And when animals that aren't native to Britain do appear, like the golden hamster in one book, they speak with foreign accents to indicate that they aren't from Mossflower.
The animal characters also gradually became more human-like, especially badger characters. In the first book, specific note is always made when Constance rears up on her back feet; in later books, even badgers are assumed to be bipedal.
The order of Redwall itself started out as reminiscent of a Catholic monastic order: the members wore habits, they lived somewhat sequestered inside their Abbey, and remained celibate for the entirety of their lives. Cornflower got yelled at for flirting with Matthias, who was a postulant of the order and therefore off-limits; when they got married, Matthias was mentioned to have left the order and lived apart from the monks. In later books, all that's left of this rule is that there is an Abbey. Even Abbesses and Abbots can be married, and not even the habit is required anymore, morphing it into some sort of peaceful commune that's little different from other communities in the forest.
The first book Redwall had more religious/mystical references, including mentions of heaven and hell and a snake named Asmodeus, after a demon in the Catholic/Orthodox bible. Again, these are toned down in establishing Redwall as its own universe. In the first book, it is also ambiguous whether Sela the fox actually had unique powers. Later in the series, any claims of supernatural powers are explicitly presented as a Scooby-Doo Hoax.
In an early scene in the first book, claims that Cluny's horde is all evil is met with cries of "That's right, give a rat a bad name!" implying that there are rats living in Redwall. This is most certainly not the case in the future.
Feast scenes in the first couple of books sometimes feature corn and tomatoes. In future books, only Old World produce is available.
The Sword of Martin is renamed "Ratdeath" at the end of the first book. This is never mentioned again.
In the first book, there are mundane animals like chipmunks and hawks in Xanth, and the animals have a Magic Talent just like most people do. The infamous puns are almost completely absent. Later on, Anthony decided that everything in Xanth either "had magic" or "was magic", and all animals and plants became of the "was magic" type.
The first few books are dark in tone, and form a genuine fantasy trilogy. Xanth started an exploration of a fantasy world where everyone has one specific talent. Then it got more parodic, more comedic, even cartoonish, and is now a full-blown Running the Asylum cash cow.
Holmes displayed some mannerisms in the first story that were then dropped — most infamously, his claim that he deliberately forgot all facts that were irrelevant to his work, such as that the Earth goes round the Sun! (That particular fact doesn't turn up again, but he displays much more well-rounded knowledge in the rest of the series. At the very least, Watson's claim that Holmes knows nothing of politics is disproven by "A Scandal in Bohemia".)
A Study In Scarlet is also the only Holmes story to cut away to an omniscient narrator, rather than staying within the confines of Watson's knowledge. Players of the Great Game do a good job ignoring this.
Holmes’s cocaine habit is directly observed (canonically) only in The Sign of Four (1890) and "A Scandal in Bohemia" (1891), two of the very earliest works in the series. Watson states he made him quit much later in "The Adventure of the Missing Three Quarter". Holmes’s cocaine habit is also mentioned in "The Five Orange Pips" (1891), "The Man with the Twisted Lip" (1891) and "The Adventure of the Yellow Face" (1893).
Bill Bryson's The Lost Continent was written by an angrier, less mature Bryson and it shows. Readers who begin with later works might be surprised at how acidic, anti-American (and arguably elitist) Bryson was before he mellowed. Certainly he seems much less fond of his home town, Des Moines, and his father, and much more of a wannabe Brit. Same goes for Neither Here Nor There, published two years after The Lost Continent. It's certainly rougher and more acidic in tone than his later books.
To avoid these sorts of situations, LucasFilm had forbidden any EU works dealing with the pre-movie lives of Anakin Skywalker/Darth Vader, Obi-wan Kenobi or Yoda, even unto not allowing writers to identify the race or home planet of Yoda. Lucasfilm didn't say anything about Boba Fett, so some novelists decided to tackle him, only to need some industrial-strength retcon when the prequels came around.
The novelization of A New Hope has the same Early Installment Weirdness problems. The story is told from the point of view of a old chronicle, numerous scenes are added that were cut from the movie ultimately (including the Jabba meets Han scene added in special editions), Earth animals are commonly mentioned, Luke's squadron is identified as Blue Squadron (which was originally intended in the film, but due to SFX limitations was changed to Red), It also portrays the Emperor as a puppet of the bureaucrats, contradicting his usual characterization. Fittingly enough, both books were written by Alan Dean Foster.
The title Dark Lord of the Sith originally comes from these novelizations, appearing nowhere in the original film trilogy. And it was used as a title belonging exclusively to Darth Vader, and is implied to refer to a planet or solar system that he rules, rather than to an ancient cabal of evil Force users.
Marvel Star Wars started coming out around the time of ANH, and its early issues show this trope. Not only did Vader and Chewie look weird, but Obi-Wan once advises Luke to use all of his anger in a Force attack on Vader, there was a world where Obi-Wan and his two apprentices, Anakin Skywalker and Darth Vader had visited, and a story about a Jedi, set in the past, had the Jedi wearing a space-fashion outfit.
The idea of red lightsabers generally being reserved for Sith didn't become established until as late as Attack of the Clones. Before that, the EU had plenty of examples of Jedi using red sabers and Sith using differently colored ones, creating a fairly jarring effect in Dark Empire when Palpatine is shown using a quintessentially good guy blue saber. The early EU also contained several lightsaber colors rarely seen since, such as yellow, orange, and pink.
David Eddings's Belgariad series starts off this way; eg. Belgarath's comment about "well then we'd have to wait another century for the circumstances to be right again" which is totally at odds with all the prophecy stuff from later books.
David Eddings's Elenium series begins with Sparhawk planning to quietly garrotte someone, which seems quite out-of-character later on; in general the first few chapters seem a lot darker than the rest of it.
The first book contains several elements that are never mentioned again, such as the ability to broadcast your thoughts to a person in morph when you weren't in morph yourself, or the psychic Info Dump laid on Tobias by Elfangor. The first one of those was addressed by the author as a mistake. She was unsure of how thought speech should work, and by the time she decided it was too late to change the first book. And apparently she went too far the other direction when she had Ax declare, in an early book, that humans couldn't use thought speech at all (even if said human was actually an Andalite in morph), which admittedly doesn't make much sense. That, too, gets retconned away when Ax uses thought speech in human form in a later book.
In the first book, Elfangor seems like a psychic. He appears to be able to feel the kids' fear as they hide, and let them borrow some of his courage. Andalites are never portrayed with such abilities again, with their telepathy limited to communication.
Interestingly, this ability reappears much later in The Ellimist Chronicles. Apparently Andalites have always had this ability, but Elfangor is the only modern-day one to use it on-camera.
Also, Visser Three acts as if he's meeting Elfangor for the first time. ("Prince Elfangor-Sirinial-Shamtul if I am not mistaken. An honor to meet you..."). Now that we know better, from The Andalite Chronicles, we'll just have to assume he was being sarcastic.
A big part of the first book involves the heroes finding out who is a Controller because said person suddenly acted out-of-character. In later parts of the series, Yeerks make a point of acting very in-character, and it's even shown that, by accessing a person's memory, they are brilliant at acting the part. This actually became an important plot point in the pilot episode of the show, as the Animorphs identify Chapman as a Controller because he maintains his habits even when no one is around, in this case, scratching his leg with his foot.
Hork-Bajir having a biological imperative to make war with each other every sixty or so years (later books indicate Hork-Bajir don't even live that long) and the warning that Yeerks have infiltrated the Andalite home world, which never goes anywhere.
In addition the early books mention several races said to be enslaved by the yeerks (i.e the ssstram, mak, and nahara) which are never encountered or even mentioned again.
The rules of magic seem to have not been quite worked out in the first book. Several spells are performed nonverbally and without wands, both of which are established to be extremely difficult in later books. Hagrid, despite having been expelled in his third year, is able to perform nonverbal spells, something which the main characters don't learn until their sixth year. He also successfully performs magic with a broken wand, something that the second book establishes is nearly impossible. Apart from the levitation spell, specific spells, their incantations, and their specific effects are not really brought up in the first book.
In the first book, Molly Weasley asks which platform they need to get to, as if Platform Nine and Three Quarters wasn't the only magical platform from which the Hogwarts Express departs every single year. (However, it's possible that she asked this to test whether Ginny knew.)
At the end of the first book, Dumbledore tells Harry that he thinks Snape was being protective of Harry due to James saving his life. Later books show that this is nonsense and Dumbledore knows full well that it was always about Lilly. And this is right after Dumbledore explicitly says he won't lie to Harry.
None of the characters, not even those who were raised wizards, seem to know how Sorting works in the first book. Fred convinces Ron that the Sorting involves wrestling a troll, even though he has three other brothers who had no reason to lie. In addition, Ron is fooled by his brother George into thinking a doggerel couplet is a spell which he could use to alter the pigmentation of Scabbers the rat, while all real spells seen in the books (after this example) are Dog Latin.
The school song, performed at the beginning of the main characters' first year, never appears in any other books. J.K. Rowling said that the school song is only performed whenever Dumbledore is in a good mood and feels like it. It's likely that with all the dark things that go on in the ensuing years, he just never feels up to it. A deleted scene in the Goblet of Fire film shows the Hogwarts student body singing it to the students from Beauxbatons and Durmstrang.
Other early books show a bit of this too, especially in terminology; Prisoner of Azkaban refers to "Hit Wizards", whose role is replaced by Aurors. At one point in the first book, Harry refers to "going over to the Dark Side" (obviously in the context of refusing ever to do such a thing). It's not until the fourth book that we learn that Voldemort's supporters are called Death Eaters and Ministry of Magic employees who hunt dangerous wizards are called Aurors.
Terminology is confused several times in early books. In the first book, Neville mentions that his family was worried he'd turn out to be a "Muggle", i.e. not have any magic, but from the second book on non-magical individuals born to magic parents are called "Squibs".
A minor example: Harry is dismayed to have lost "two points for Gryffindor in his very first week," both of which Professor Snape deducts in Harry's first Potions class. He will lose many, many more during his years at Hogwarts. Points in general seem to undergo inflation as the series goes on; where teachers took one or two points away for infractions in the first book, they commonly take five or ten for similar offenses in later books. Some deleted footage from The Film of the Book reflects this: Snape deducts Gryffindor five points when Harry can't answer his questions and refers him to the obviously much more eager Hermione.
There's also the Deluminator, which is only called such in the last book. When it appeared in the first and fifth, the narration referred to it as Dumbledore's "Put-Outer".
In a non-continuity-related example, J. K. Rowling's writing style becomes less Beige Prose-ish, and replaces more and more of its Roald Dahl bounce with sturm-und-drang, as the series goes along. As an example, in trying to get away from the constant owl post deliveries, Vernon moves his family and Harry to a rickety cabin perched on a rock in the middle of the sea, an abrupt change of scenery that wouldn't be out of place in James and the Giant Peach. After Hagrid tracks them down, this cabin is never mentioned again.
The constant owl deliveries are themselves odd, when one considers that Dumbledore has actually visited Petunia in the past to convince her to keep Harry, and has access to several other means of communications he's not otherwise shy about using. It feels like the Dumbledore of the later books would've sent Hagrid out to fetch Harry way sooner, or immediately escalated to a howler to remind Petunia of her obligations.
Early in the first book, Petunia claims Lily turned teacups into rats while she was home from school. Later in the same book, we find out Hogwarts students aren't allowed to use magic outside of school; Deathly Hallows confirms the rules haven't changed since Lily went to school. Rowling's answer has been that Petunia was exaggerating but Lily occasionally got warnings for out-of-school magic.
A certain character in Chamber of Secrets mentions Hagrid was raising werewolf cubs under his bed. In the very next book, it becomes clear that werewolves are humans afflicted with a disease that makes them become wolves during a full moon. JKR later said that the character in question was lying, which would be in-character, but is not made clear at the time. Perhaps as a callback to this, in the seventh book the same character refers to the hypothetical offspring of an heroic werewolf as "cubs", in a context that makes it clear that he's making a rather offensive joke.
The "closeness" of the Muggle and wizarding worlds, in terms of how much the wizards knew about Muggles, seemed to be more fluid in the first book. In The Philosopher's Stone, Malfoy is overheard telling stories about flying that always seemed to end with him narrowly dodging Muggles in helicopters. Even in the second book, it would have been a point of pride for Malfoy that he would have no idea what a helicopter was (although it's possible he might have been lying in order to make himself look good in the first book).
The first chapter of the first book uses a classic third-person omniscient style of narration, starting in Vernon Dursley's head before casually switching, around mid-chapter, to the point of view of a cat who turns out to be Professor McGonagall in disguise. It also jumps to Ron and Hermione's point of view during the first Quidditch match. Later books are almost exclusively from Harry's point of view, only occasionally going to a different Po V during the opening chapter. Even then, the non-Harry chapters are done in strict third-person limited.
Snape's introductory statement that "There will be no foolish wand-waving or silly incantations in this class" implies that there is some sort of disciplinary rivalry between potion making and wand-based magic, however this is never elaborated on; the two areas of magic are shown to be mutually complimentary, with good potion makers uniformly depicted as being expert spells-casters as well.
The first two books have a lot of world building details that are, for lack of a better word, "weirdness for weirdness' sake". Things that are just plain odd or borderline nonsensical because it's a world about wizards and wizards don't do logic. Things like the wizard money and its logic-defying subdivisions (1 Galleon = 17 Sickles = 493 Knuts. Sickles and Knuts go almost unmentioned after the first book), Platform 9 and 3/4's numbers such as opposed to something more logical (9 and a half? One above the highest existing platform?). Later books, while keeping the magic elements, tone down the nonsensical weirdness to have the world be more internally consistent.
Stephen Fry's narration on the audio books is slightly different for the first book too. Notably he gives Harry a very timid voice that he doesn't have in any of the other audio books. He also pronounces Malfoy's name a little differently - as "mal-foy", with the emphasis on the second syllable. In other audio books he pronounces it as "mal-foy".
The first book has a line from Scathach that implies she witnessed Joan of Arc's death, and there's another line that alludes to Joan dying young. The very next book has the protagonists meet Joan herself, who has been alive and well for hundreds of years. And Scathach claims to have personally saved her life during her execution, making the line from the first book quite odd.
The first book said that the fifth element is Time. No other books reference a fifth element, and the twins only get instructions in the traditional Four-Element Ensemble.
Reading the first edition of the Brothers Grimm's Fairy Tale collection can be quite shocking for many people who are used to later editions. The stepmothers of Snow White and Hansel and Gretel are instead their biological mothers, Cinderella does not go to the first ball (instead watching it from the pigeon roost) and only goes to the second and third, and many tales of French origin appear that don't appear in later editions. In addition, many stories in the first edition are extremely dark, such as "Two Children Play at Slaughter", where one child straight up murders another while playing. These dark stories were mostly cut out of later editions.
Steven Erikson's Malazan Book of the Fallen series shows marked differences between the first book, Gardens of the Moon, and the later books. (Examples include Tool's hundred-mile-diameter magic-deadening Tellann aura and the interaction of munitions with magic use, among many others.) To Malazan fans, this is known as a GotMism. The first book was written around a decade before any of the other ones.
Most people who've read The Wonderful Wizard of Oz these days know little of the numerous sequels that followed. They'd be shocked at how many of the rules were rewritten even between the first and second books. Issues such as the existence of money, the actual number of witches (and related magic users) in the land, whether or not the Emerald City is actually green, and if people can actually die were altered drastically over the first few sequels. No doubt this is because while the rules found in The Wonderful Wizard of Oz can work within the plot of that particular story, they proved too confining when others had to be written.
The Hobbit seems like a somewhat fanciful children's tale compared to The Lord of the Rings; Tolkien hadn't even decided it was going to take place in the context of his wider mythology when he originally wrote it. Though modern editions have removed some of the weirdness, earlier versions had oddities like a railroad in the Shire, and Bilbo musing about taking a trip to China. Also the Orcs are referred to as Goblins, with the word Orc only appearing twice. Secondly, many of his earlier drafts for his mythology have been published, and if anything they seem more fanciful still. By the way of an example, in one version Sauron's role in the story of Beren and Lúthien is played by an evil cat, and Beren is an Elf. Aragorn was originally a hobbit named "Trotter," so-called because he wore wooden shoes to appear taller, and Treebeard was a villainous giant rather than a friendly ent. Balrogs were much more numerous and less powerful than they later became. Whereas in the final versions killing a Balrog has only been accomplished thrice (except in the final War of Wrath) and each time it was a Mutual Kill, early drafts had the heroes killing them by the hundreds.
The Hobbit and the early portions of The Fellowship of the Ring are narrated in a chatty and snarky tone that sounds like the narrator from obvious children novels like Narnia or Mary Poppins; especially when compared to the "ye epic of olde" cadence of The Return of the King. This is because Tolkien originally wrote the Hobbit as a simple children's book, and The Lord of the Rings was (ironically, considering its current length) going to be a short sequel to that story. But the story "grew in the telling" as Tolkien said, and so the early parts of The Fellowship of the Ring, written while Tolkien still thought he was writing another children's book, seem far less solemn than what comes later. From a literary perspective this has the writing becoming more serious as the story becomes more serious - an interesting effect.
The Last Battle states that there are only two god-like beings in Narnia: Aslan and Tash. LWW, however, shows that Bacchus and Silenus occasionally dropped into Narnia to party with the fauns. Prince Caspian even mentions Pomona, the Maenads and a River God serving Aslan. These are later dropped as the series progresses.
The first book also had a more whimsical feel to it in regards to the Fantasy Kitchen Sink meets Anachronism Stew tone, what with Mr Tumnus's umbrella, Mrs Beaver's sewing machine and the infamous appearance of Father Christmas. Modern appliances weren't around in the later books.
In LWW much is made of the fact that there are no humans in Narnia prior to the arrival of the Pevensie children; Tumnus is even mentioned to have a book titled Is Man a Myth? on his bookshelf. This is a big deal particularly because of a prophecy declaring that a human will herald the end of the White Witch's reign. In The Horse and His Boy, however, we learn that there's a whole kingdom (Archenland) populated by humans right next door to Narnia.
In the first Star Trek: New Frontier book, written in 1997, prior to the trend towards consistency in the Trek novels (which started around 2001)), Danter is said to be a member of the United Federation of Planets. This seems rather odd, seeing as they're most definitely not what we'd expect from the Federation. Indeed, they're openly imperialistic. Later books seem to have retconned this, making Danter definitely an independent nation.
Over Sea, Under Stone, the first book of The Dark Is Rising, is a rather standard, almost Enid Blyton-esque children's adventure story, with less of the fantasy elements and references to Celtic mythology that defined the later books (they only show up in the second half of the book, and even then they're relatively subtle).
The first Fablehaven book is a mostly cheerful, good-natured, and lighthearted fantasy story about a pair of bickering siblings who discover that their grandparents run a nature preserve for magical creatures. The entire concept is treated with wonder. The later books in the series, however, are extremely dark, changing their tone entirely about what it's like to work in a magical preserve, and don't shy away from violence and death. The entire thing becomes a serious Crapsaccharine World.
The first book of the Dragons/Last Dragon Chronicles, The Fire Within, is massively strange in comparison with the rest of the books in the series. The main antagonist is Henry Bacon (who is a grumpy good guy in every other book), and book has a very simple and lighthearted plot about trying to save a one-eyed squirrel from a crow—with some Reality Warping thrown in. The rest of the series, however, is a dark, heady series about mankind's destiny, human nature and the nature of God, quantum physics, and what can only be described as the dissolution of reality at the hands of a group of superbeings. On the side, it also addresses issues like the nature of adultery and the ways in which we cope with grief. The author eventually started a spin-off series that was much closer to the original book, presumably to avoid warping the innocence of children further.
Magic: the Gathering pre-revision lore from novels, comics and magazines mostly fit with modern canon, but there are plenty of details that are overwritten by later sources. 
In the Nick Velvet stories by Edward D. Hoch, Nick was originally billed as a 'Thief of the Unusual' rather than a 'Thief of the Worthless'. Several early stories feature him stealing items that definitely have a monetary value. The first story has him stealing a rare tiger from a zoo. It was several stories into the series before Hoch settled on the only stealing items with no value aspect that made the character unique.
Storm Front, the first book, matches the tone of the later installments fairly well, but refers to some world-building concepts that were changed in later entries of the series. For instance, there's a reference to vampires being unable to enter homes uninvited because they're creatures of the Nevernever, and need to expend constant effort to maintain corporeal form, and crossing a threshold uninvited blocks power. The threshold-blocking-power bit is maintained in later books, but vampires are later established to not be from the Nevernever, and only some breeds really have to worry about invitations. Also, there's a reference to a singular Queen of the Fae, while late installments establish two separate Fae courts, with three Queens each.
There's also the issue that in the early books, the author did not know the Chicago area very well, resulting in various errors in geography and architecture. This was remedied later after Butcher did his homework.
Shockingly, Storm Front boasts entire chapters in which there are no pop culture references. Even finding a page without any would become rare by the third or fourth book.
In Fool Moon, Harry summons the demon Chaunzaggaroth. His conversation with him makes it very clear that Chauncy is supposed to be a fire-and-brimstone Dante's Inferno demon, with it referring to Saint Patrick and the Catholic Church as "the other side." But in Death Masks he tells Murphy that "demons" are just harmful spirits from the Nevernever and that the Fallen which empower the Knights of the Blackened Denarius are the only true Biblical enemies-of-God demons that are active in the world.
Later books also establish that some regions of the Nevernever closely resemble the various underworlds of human myth, so it's possible that Chaunzaggaroth simply comes from a region that mimicsFire and Brimstone Hell, and deliberately milks that cliche in his conversation with Harry.
The first three or even four books focus almost exclusively on cases Harry takes in his job as a wizard-for-hire; a sort of magical Private Eye. In fact, that was meant to be the spirit of the books; a sort of Raymond Chandler meets JK Rowling. Elements like the Vampire Courts, the land of Faerie and the White Council were certainly there, but far in the background. Then book two added the Alphas, book three added the Knights of the Cross and expanded a bit on Harry's background, and book four added the Faerie Queens and the entire world therein, as well as expanding greatly on the White Council and its function in Harry's world. After that, Harry's work as a wizard-for-hire took a back seat to his war with the Vampire Courts, his ever-expanding supporting cast, the Fey and their influence in the real world, political intrigue and Manchurian-Candidate-style goings-on in the White Council, etc, all building on a previously barely-hinted at Myth Arc that directly led from book to book.
The chief differences are made manifest by watching the television adaptation of the books. The series was a loose adaptation to begin with, but went into production when only six books had been published, and heavily based their stories on the tone of the first three books. In the series, as with the first few books, the focus is on Harry as a special consultant to the Chicago Police, which is also true of the first few books, wherein he works with the police so often that he considers several of them to be his personal friends. Later, his only contact on the force is Lt. Karrin Murphy, who mostly is shown helping him with his problems rather than the other way around.
And after the aptly-named Changes, the stories shift yet again. Literally nothing is the same afterward, with the possible exception of the series still mostly taking place in Chicago, and keeps getting more different with each volume.
"Extricating Young Gussie", the first of P. G. Wodehouse's short stories containing Jeeves and Wooster. Bertie's personality and his relationship with Aunt Agatha are all in place, but their family name appears to be "Mannering-Phipps" instead of "Wooster". More noticeably, Jeeves appears for all intents and purposes to be an ordinary valet, and when Bertie gets in trouble and needs help, he has no idea who to ask. The surname "Wooster" and the personality of Jeeves as we know him today don't appear until the second story, aptly titled "Leave It to Jeeves".
Mercedes Lackey's Dragon Jousters series begins with a slave-boy raising a dragon. It ends with kings and the resurrection of a dead city and evil wizards. The first and the last book don't even seem like they're in the same series.
The early Goosebumps books could get surprisingly dark. They also didn't have every chapter end in a Cliffhanger as most of the books tend to.
Historia Brittonum, the earliest work offering an account of the supposed career of "King" Arthur. Only that Arthur isn't a king but a general; a certain Ambrosius is considered the most glorious king of the Britons instead; and there's no Merlin, no Sword in the Stone, no Guinevere, no Table Round, no Grail, no Excalibur, and no knights. There's essentially nothing of what defines Arthur in the writings of Chrétien de Troyes and Thomas Malory.
The first Gaunt's Ghosts book is near unrecognisable compared to the later ones. Instead of Loads and Loads of Characters, there's a handful of core cast and a bunch of Red Shirts. Instead of the Ghosts being an elite unit that take few casualties compared to other regiments, they die like flies. Instead of swift, dynamic battles, the first battle is trench warfare. note Indeed, when trench warfare reappears five books later, it's because of the incompetence of the local government and the Guard officers take every opportunity to note how bloody stupid and wasteful it is. Instead of the top brass ranging from decent to less-than-competent, the Ghosts' superiors are outright malicious and spiteful, along with an Inquisitor. Even a few Chaos Space Marines appear and are killed (relatively) easily, yet almost never appear in other books and when they do they're The Juggernaut. This is easily explained by how the Imperial Guard in Warhammer 40,000 originally were portrayed as helpless Cannon Fodder led by idiots and maniacs, but Gaunt's Ghosts transformed their perception and their portrayal into a genuine Badass Army.
Dave Barry's work in the 1980s was noticeably tamer, less biting, and less dependent on his myriad of Running Gags. It was around Dave Barry Slept Here that his Signature Style hit full steam.
His first four books also had illustrations by Jerry O'Brien, which were much more cartooonish and wacky. Most of the subsequent books had Jeff MacNelly (creator of the comic strips Shoe and Pluggers) until his 2000 death, when Gary Brookins replaced himnote also doing likewise on Shoe and Pluggers, although he actually took over the latter well before MacNelly's death. One of the exceptions was Slept Here, possibly because it had maps instead.
The first few installments of Uncle John's Bathroom Reader were much smaller (the first one was only 224 pages) and its articles were far shorter — usually a two-to-three page summary on a topic, followed by a bullet list of trivia. By Uncle John's Ultimate Bathroom Reader, the average size of the book more than doubled as the articles became longer and more in-depth. Eventually, the writers even started including multiple-part articles spread across the book, and an "Extended Sitting" section composed of double-length articles. Their research and copy-editing have also improved significantly.
The first book, having the burden of introducing the world of Classical Mythology to newcomers, goes to great lengths to point out that Greek Mythology is different from Christianity. These include scenes such as Percy being told not to worry too much about the possible existence of a "Capital-G god," and Percy commenting on seeing a televangelist in the Underworld (Annabeth tells him that the Mist probably makes him think he's in Hell). After the first book, Christianity is rarely (or not at all) mentioned.
Also, in the first book and parts of the second, Percy would usually refer to gods by family names, such as Percy calling Hades "Uncle." By the time the third book rolls around, this is almost completely dropped and gods are treated with much more respect, normally being called Lord or some other official title.
The four Wardens (of the four cardinal directions) seem early on to be very important positions, with Ned objecting to Jaime Lannister being named Warden of the East (because he will eventually inherit his father's place as Warden of the West as well). Later on this seems to be not much more than a title, with the lords' power determined by their family and alliances, and it is firmly established that members of the Kingsguard like Jaime cannot hold lands nor titles.
The first book is also noticeably more tightly plotted, with few of the increasingly numerous characters and side-plots of the later books. The reason for this is simple; it was originally conceived as a trilogy with a specific arc for each book, and later was expanded into the currently planned 7-book series.
Tyrion is introduced sitting above a doorway, which he then flips off of, landing gracefully in front of Jon Snow. GRRM later learned that a man with dwarfism could not be capable of this; consequently, Tyrion is described as clumsy in later books and this event is never referenced again.
The first three books identify all chapters, with the exception of the prologues and epilogues, by the first name of the POV character of each chapter. From Book 4 on, GRRM began to switch these out in some cases in favour of descriptive titles of the POV characters.
The short story, Weyr Search, that became the opening of Dragonflight and launched the Dragonriders of Pern series, has a number of differences from the rest of the book (written years later), and the book has marked differences from the rest of the series. The biggest oddity of Weyr Search is that Lessa has an explicit projective empathic ability that works on humans; she uses it on F'lar. Of note from the book to later novels, the dragons aren't given actual lines — the narration paraphrases their speech to their riders (as with fire lizards later), making it seem more empathic and less explicitly telepathic.
The naming conventions of Dragonriders changes between the first book and the rest of the series. In Dragonflight, Lessa and F'lar discuss a newborn baby whose name has the familiar "consonant-apostrophe-three letters" combo. In later books, it's established that male Weyrfolk are given a name that's a combination of the mother's and the father's name, which is changed to a shortened version if they become Dragonriders. For example, in Masterharper of Pern, young F'lar is called Fallarnon.
The Warrior Cats series has several, especially in the first book:
Each Clan keeps mostly to itself and doesn't know much about what's going on elsewhere in the forest, and the other Clans' territories are generally a mystery to them. Tigerclaw freaks out in the first book when they take Yellowfang as a prisoner because he thinks that now ShadowClan will know where the ThunderClan camp is, and that they'll need to move, while in later books (even ones that take place around the same timeframe) they actually visit each other's camps on a fairly regular basis. Likewise, in the first book, they say a few times that "that cat must have been killed by an enemy patrol" and just forget about it - most notably the ShadowClan leader's death - when later on such an event would be huge news and they would actually confront the other Clan and try to figure out what happened. This is probably partly because since the first book they've established exact rules for the previously-vague warrior code, one of which forbids killing except for rare circumstances.
Their terminology changes a bit, such as the way they use hyphens and capitalizations in words (i.e. "Clan mate" becomes "Clanmate"). Also, in the first book they use the word "queen" for any female cat, but later on it only gets used for females currently pregnant or nursing kits, and "she-cat" becomes the general word for "female".
The Clans are a lot more formal early on; cats refer to parents by their actual names rather than calling them "mother" or "father", and there is almost no use of slang such as "cool". The Gatherings in the first series tended to be different groups of cats mixing and talking about current events - for example, a senior warrior talking to a group of apprentices, or a young warrior joining a bunch of elders and medicine cats in a discussion, while lately it's mostly cats sticking with their own age group and gossiping.
Mates aren't, for the most part, treated as major relationships in the first series, and it is mentioned that warrior fathers don't stay close to their kits (with one or two rare exceptions). In the third series, one cat who had kits as the result of a one-night stand (which in and of itself is rare later on) is criticized because he doesn't want to play with his kits, and in the third and fourth series especially, everyone gossips about young couples and young cats talk about who they like.
Early on, cats are rarely mentioned by name unless they actually have speaking lines in a scene, and sometimes not even then. In The Prophecies Begin you're likely to see something like "Tigerclaw and a patrol of warriors were leaving the camp" while in Power of Three or Omen of the Stars it would be more like "Bumblestripe, Thornclaw, Cinderheart, and Ivypaw were leaving the camp on a hunting patrol". Cats at gatherings and in battles are also usually just described as, for instance, 'a silver tabby' or 'a dark queen' more often than they are named. Kits get this treatment, too, leading to the odd effect of cats not seeming to have names at all until they're apprenticed.
The timing of cats' apprenticeships is weirdly off in series one, and their general ages. This is sometimes due to retcon, but it still leaves The Prophecies Begin arc very weird timing-wise, especially when it comes to kits and apprentices. Ravenpaw refers to Sandpaw and Dustpaw as being longer apprenticed than he, Graypaw, and Firepaw, despite the fact that he and Dustpaw are littermates and should have started training at the same time. Fireheart and Graystripe become warriors before Sandstorm and Dustpelt, which isn't that odd considering they'd just done something heroic, but they'd only been training for three moons at most. Thornclaw and Brightheart start their apprentice training in the book after their siblings Brackenfur and Cinderpelt do, for no discernible reason.
Early books in Sword of Truth are much more stock fantasies. The most common criticism of the early books was that Goodkind was ripping off The Wheel of Time. It's not until the fourth book that Goodkind began adding his Objectivist philosophy and Author Filibusters, and the series took a much different direction.
The Horatio Hornblower books are written in Anachronic Order. Lieutenant Hornblower, published seventh, is the story of how Bush met the title character when they were both young lieutenants. This retcons The Happy Return, published first, where Bush apparently met Captain Hornblower for the first time at the start of the Lydia's voyage.
In the third novel, Sojourn, of The Dark Elf Trilogy, we have Bruenor Battlehamer's constant desire to eat Roddy McGristle's dog. We're never given a reason why he wants to, he just... wants to eat dog-meat. It's not even implied that this is something normal for dwarves in the setting. At the story's end, he gets to enjoy a roast dog's leg. Ironically, that he never thinks this way again is actually justified; after he finally gets to eat the roast dog meat, he is quite horribly sick, vowing he'll never eat dog again.
It does become something of a Running Gag in the later books that Bruenor has a bizarre fascination with disgusting culinary experiments. Once he even makes stew out of an ogre's brain!
Oddly enough, in one of the later books a short scene with a seemingly-supernatural entity comes up and seems jarringly out of place with the series' new tone, even though it's significantly toned-down from the supernatural elements of the first book.
Arly Hanks has a deputy working for her in the first of Joan Hess's Maggody mysteries, and the sheriff's office largely takes over the investigation into Jaylee's death, in contrast to later books when she has no underlings and is railroaded into do all the grunt-work because Sheriff Dorfer is out fishing or watching baseball. Mrs. Jim Bob actually chews out her husband for hassling Arly, rather than sniping at her in Holier Than Thou fashion, and Jim Bob himself seems genuinely fond of fishing and hunting, rather than using them as a cover for his philandering.
The Rainbow Magic series has this. The first series of books took a while to establish Kirsty and Rachel before sending them to Fairyland, the goblins were built up as major threats, they had to find the fairies themselves rather than magical items, and Jack Frost was captured at the end and nearly melted until the fairies relented.
It was also very clearly meant to be a standalone series. The Weather Fairies series was only made due to its popularity.
The Tortall Universe has been subject to some Continuity Drift. In the first quartet, the narrative departs from Alanna's POV far oftener than the subsequent series (we've hardly ever seen the villain's POV again). She also tangles with demonic entities and "elementals" which so far haven't shown up again, although the Ysandir did get a namecheck in the Provost's Dog trilogy.
Amelia Bedelia readers accustomed to Lynn Sweat's character designs (seen in almost all of the books written since 1976, as well as the TV Tropes page Literal-Minded) might find it jarring to read a book illustrated by one of his predecessors, especially if it shows Amelia Bedelia with no stockings and/or Mr. and Mrs. Rogers (her bosses) with gray hair instead of brown. The first book also portrays Mr. and Mrs. Rogers as a wealthy couple with a mansion, while later books inexplicably relocate them to the suburbs.
The first three books barely make note of the parabatai bond. Strange, considering how important it is in the prequel series. The second half of the series compensates by emphasizing the bond several times.
The Forsaken are only significant in City of Bones. After that, they are barely mentioned at all.
Drinking Midnight Wine, although indisputably part of Simon R. Green's Greenverse, is a stand-alone full-length novel with more of a sober, mainstream feel than his later Urban Fantasy series (Nightside, Secret Histories, Ghost Finders), which are quick romps that don't take themselves so seriously. Moreover, Midnight Wine depicts the realm of magic as its own parallel reality (Mysterie) which is difficult to access from our own (Veritie), with science being little-known in the former and magic, virtually inoperable in the latter. By comparison, magic and the supernatural are fully integrated with our world in his series works, just kept hidden from the masses, and Mad Science or Magitek are as potent of an influence as the mythical.
The first Madeline book is a realistic slice-of-life story about the titular little girl having her appendix removed and her friends visiting her in the hospital - a plot line that doesn't even appear until halfway though the book, the first half being devoted to introducing the characters. If it weren't such a famous book in its own right, it would feel very strange compared to the rest of the series, which is much more plot-driven and adventure-oriented, with the later entries even introducing magic and fantasy elements.
The magic system in The Wheel of Time series isn't really codified until the third book, and so the early books show people using the One Power (magic) to do some things that don't fit with later applications as well as some contradictory rules.
Moiraine claims that her Magic Staff and tikka-like jewel are mere "aids to concentration" and that objects can't have intrinsic power. In later books, ter'angreal are revealed that are quite powerful even without a channeler as well as enabling channelers to do things they'd be normally be unable to do.
Moiraine says that her bond with her Warder can't tell where he is but, in later books, both Aes Sedai and Warders know where their bonded half is (at least a direction and vague idea of distance).
At one point, Moiraine uses her staff to misdirect followers by casting their scent and footprints in a different direction. While moving air itself is shown to be fairly trivial, creating a long enough trail of footprints to fool followers doesn't seem like something that would fit with what she should be able to do.
Moiraine also creates massive walls of flame and severe earthquakes to combat a large group of enemies; this level of power isn't displayed later and her fighting with the One Power is limited to hurling single balls of fire and spontaneous ignition.
Moiraine creates a "bending" to render the party invisible and calls it a "simple feat". It's later revealed to be a lost art. Moiraine's ability to do this without other Aes Sedai knowing is partially handwaved once the ability is "rediscovered" by noticing that some Aes Sedai discover abilities and keep them to themselves.
In later books, any channeler (of the appropriate sex) can passively see weaves made by other channelers but none of the channeling characters see weaves until the channeling is developed in the later books.
In the early books, Moiraine claims that being a channeler provide an intrinsic protection (as well as an aura of protection to others nearby) from evil influence as well as granting the ability to sense evil. None of the channelers later show this intrinsic resistance and have to rely on active "wards" to keep evil parties from influencing them.
Max and Ruby: The very first Max and Ruby books by Rosemary Wells from 1979 (Max's First Word,Max's New Suit,Max's Ride, and Max's Toys) were all very small compared to later books from the series. Especially Max's First Word where Max's overalls were colored white and the book was only 6 pages long. This would later be used for the "Baby Max And Ruby" series which is geared towards very young children.
Famously, the beginning of Fifty Shades of Grey is about Ana going to interview Christian for the college newspaper. For the rest of the series, Christian is careful to avoid the press.
Maura Isles doesn't even exist in The Surgeon, which is technically the first book in the Rizzoli & Isles series, and when she shows up in the second book, she's a minor character with only a few scenes. Rizzoli herself is a secondary character in the first book, and a thoroughly unlikable one at that—brusque, abrasive, almost pathologically jealous of beautiful women because she herself is plain and average looking, so much so that she spends most of the book treating the protagonist/titular character—a rape victim being stalked by a copycat of her assailant—like garbage. But somehow, she got such positive feedback that author Tess Gerritsen nixed plans to kill her off and begun the series of books based on them, alternating between which woman takes the lead role in each novel or splitting it evenly between them.
The Gunslinger, the first book of The Dark Tower, is very different from the later installments, being a fix-up novel made of five short stories, giving it more episodic structure than its successors. It focuses on setting the tone of the post-apocalyptic Mid-world and as a result features very sparse dialogue and a generally somber tone, while the later books tend to favor character interaction and plot progression over melancholy world-building. This is a case where there's not a clear consensus over whether or not the early installment weirdness is a bad thing or not; many fans consider The Gunslinger to be the best book in the series because of the elements that make it stand out from the others.
The eponymous Haruhi Suzumiya seems more unhinged in the earliest light novels. In the first book, she is jealous enough of Kyon to subconsciously try to destroy the world after Mikuru gets close to him— note that Kyon didn't even notice anything amiss. In later books, she would tell him off if it looked like he was spending too much time with the other girls in the Brigade, but the world wouldn't be in peril or anything. In "The Boredom of Haruhi Suzumiya", Koizumi, Mikuru, and Nagato are outright worried that she will destroy and recreate the world because they were losing a baseball game, which is a little extreme in retrospect. It is not entirely clear whether this is Character Development or just this trope. Also, in The Sighs of Haruhi Suzumiya, Haruhi is capable of causing changes in reality without even a Closed Space, which never happens again.
For that matter, after Mikuru is traumatized for the first time, she asks Kyon if he will take her if she is ruined for marriage. Later novels will establish that time travels are prohibited from having any relationships with denizens of other times. Cue the Epileptic Trees that she somehow didn't become a time traveler until after then...
In the first book of The Traitor Son Cycle, there's a mention of a young dragon's head being one of the trophies in the Harndon castle's hall. From book two onwards, however, dragons are established as being Physical God-level creatures who are very limited in number (there's less than ten of them), so killing one should be a bigger deal. Then again, it might be that in-universe, someone mistook a wyvern for a young dragon.