In the Kenzie and Gennaro Series, the identity of the serial killer in the second book is mentioned repeatedly in later books. He essentially becomes Patrick Kenzie's boogeyman, with his memory constantly haunting his nightmares. It's easy to forget that in the second book, he was introduced as Gerry Glynn, the retired policeman who runs a bar in Patrick's neighborhood.
Also, the sixth book is a direct follow-up to Gone Baby Gone, the fourth book in the series. The conclusion to that one, where Kenzie rescues the girl from her loving kidnappers and returns her to her neglectful drug-addict mother, is spoiled on the back cover of the sixth book.
Twilight: Edward is a vampire. You can glean that from the back of the book. This despite the fact that it tries to keep the reader guessing what Edward's deal is for about the first half of the book.
It's really more of Dramatic Irony than "keeping the reader guessing"; the reader knows, but Bella still needs to figure it out.
And then book two does it again with Jacob being a werewolf. Possibly the reason that "Eclipse" has a slightly better reputation even among the Hatedom is that it's a big relief to not spend the whole first half waiting for a twist that everyone already knows.
Certain editions of the Discworld book Guards! Guards! contains character summaries of the "Duke of Ankh, Commander" Vimes, and "Captain" Carrot. For those who don't know, this is the first book of the Watch series, and it ends with a still-drunken Captain Vimes, and a still-naive Lance-Corporal Carrot.
By the way, the character summaries of these editions are found all the way back in the first book of Discworld, which doesn't even have the City Watch. In fact only four of the seventeen characters in the summaries are even in the book and only two of those played a major part.
The Harper Torch printings of the older Discworld books tend to assume you've read them already, so they tend to have fairly spoileriffic images on the cover. To their credit, the spoiler usually doesn't make sense until you have read the book, but it's still not cool to put the gonne on the cover of Men at Arms. (Not a huge spoiler though, as anybody in Roundworld rather than the Discworld will know what the weapon was as soon as the first death occurs. Any cover image or blurb that shows a plot element is equivalent, since you wouldn't otherwise know about the book until you started to read it.)
The "classy" Corgi reprints have black covers with something symbolic or significant (e.g. vampire teeth for Carpe Jugulum). The one for Feet of Clay is a bit of a giveaway◊ for a book that even calls itself a "howdunnit".
Not only does this happen with the endings of the Discworld books, but it will automatically happen if you read the first book of a series published after any earlier work. This is most glaring with the first Moist von Lipwig book, Going Postal, which includes spoilers to nearly all of the city watch books and The Truth. As this is one of the most popular novels in the series, and one of the more recently published, it is a real problem for new fans unsure of where to really start.
The Reveal at the ending of The Wheel of Time's first book? Rand is the Dragon Reborn. The cover of the third book? A triumphant Rand, with the words "The Dragon Reborn" written in big, bold letters.
The front cover art of The Moment of the Magician by Alan Dean Foster spoils what is clearly written to be a surprise, that the new evil magician in town is a kid's party magician who stumbled in from our own world, and now his lame magic works.
Haven't read the first three books in Chris D'Lacy's Dragons series? The blurb on the fourth book doesn't seem to care, since it reveals right on the inside that David, the protagonist of the first three books, dies at the end of the third book.
The blurb for the Kyell Gold novel Isolation Play blatantly spoils Dev coming out of the closet on live television, which was the climax of the previous book, Out of Position.
The blurb for the third book, Divisions, spoils the exact thing in the first sentence of the blurb. You have to wonder if Kyell is doing this intentionally as a way to tell the fans that they should know this already.
In some parts of the Warrior Cats fandom, something only counts as a spoiler if it happened in a book that came out less than a month ago. If the book is more than a month old, well... too bad. There's a good reason for that, though: even names can be spoilers; for instance, most fans call Bramblestar by his leader name since it is the character's name now, even though it took the series 24 books (not counting all the side books) to reach that point, and it means that the one main character Firestar has died, so it's a pretty major spoiler. The phenomenon also appears in the books themselves:
There are several spoiler titles, such as the Tigerstar and Sasha manga (ignoring the name thing, it's a spoiler for Moonrise).
And, of course, the character lists in the front of the books, which are so riddled with spoilers it's a wonder they still put them in the front of the books. If a character dies sometime in the book, they are treated as dead.
In the Blue is for Nightmares series, Jacob dies at the end of Silver is for Secrets. This is helpfully revealed on the back of the sequel Red is for Remembrance. Of course, the back of Black is for Beginnings helpfully notes out that he actually was Not Dead Yet.
First words on the back of the second Tawny Man book by Robin Hobb? Nighteyes is dead. Thanks a lot.
French translation of the title of Lois McMaster Bujold's first book in the Vorkosigan series, Shards of Honor, was Cordelia Vorkosigan, calling the heroine by her married name, said marriage occurring at the very end of the book and being quite hard-won by that time.
And in Cordelia's Honor, Barrayar and Shards of Honor packaged together, the blurb on the back spoils the first half of the book for anyone who was new in coming to the Vorkosigan Saga.
Because the whole plot takes off from it, the inside dust jacket (and subsequent paperback covers) for Sidney Sheldon's Memories of Midnight had to spoil the Twist Ending of The Other Side of Midnight: Catherine didn't die, but Constantin had her rescued and hidden so Larry and Noelle would be tried and executed for her murder. It's worth noting that the book was intended to work as a standalone novel as well as a sequel, via extended flashbacks to what happened in its predecessor.
There is a pulp horror novel published in 1978 and written by David Anne called Rabid, about the rabies virus spreading to the previously (and actually) rabies-free British Isles. The back-cover copy on the paperback edition ended with: "And when the virus mutated, became airborne, all mankind would know what it meant to be RABID!" The element of the virus mutating into an airborne form was a twist ending on the last page of the book and it had nothing to do with the book's primary plot.
Don't read the blurbs on the back of the later Codex Alera books if you want there to be any surprise when The Reveal rolls around about Tavi's parentage. It's much more entertaining when you piece it together over the course of 3 books, but it's very difficult to provide even a basic plot summary of the last two books without giving it away.
Hell the titles of later books give away previous plot developments. They follow the pattern of Tavi's position during that book, thus spoiling his career advancements.
So you're reading the first book of the Honor Harrington series, you come to the very end, turn the page, and staring you right in the face is an excerpt the first chapter from the eleventh book in the series that goes and spoils just about every plot point related directly to the main character.
In The Parasol Protectorate, the back cover blurb for book 2 gives away the plot twist at the end of book 1, and the back cover blurb of book 3 gives away the plot twists for books 1 and 2. Tough luck for those who want to read the entire series at once.
The second paragraph of the blurb for Inkdeath starts, "The fire-eater Dustfinger is dead." Okay, we know the movie and the book end differently, but come on!
The first book of the A Song of Ice and Fire series ends with the execution of Ned Stark and the fallout from this event fuels the plot of the next book; this, along with the fact that Daenerys managed to hatch the fossilised dragon eggs, is spoiled in some versions of the blurbs of later books. The fact that the fifth novel is called A Dance with Dragons probably also gave this away. Other major events throughout the series are also spoiled in this way, including Jaime and Cersei's incest, Robert's death, Stannis' defeat, Joffrey's death, and many more.
So, you decide that you are interested in reading the Skulduggery Pleasant books. You glance at the fifth book and decide to look at the blurb. Congratulations, the blurb has spoilered for you the huge twist at the end of the fourth book, which reveals that Valkyrie is Darquesse.
Minor example in Old Man's War by John Scalzi. It's obviously supposed to be a surprise to the reader what the Ghost Brigades really are. The first time the protagonist hears the expression, he thinks it's a joke. Later, he learns that they exist, but thinks the name is just a nickname. However, since that very term is the title of a later book, if the reader is already aware of Scalzi's other works when reading Old Man's War, it is glaringly obvious that this will be important later. And the context of its first appearance gives away what it is.
Present in various blurbs of later books for The Dresden Files: Thomas being Harry's half-brother and Molly becoming Harry's apprentice are some of the most-used ones. However, given the ending of Changes (Our Hero Is Dead), it is flat-out impossible to talk about the plot of the next book, Ghost Story, without giving away the last few pages of Changes.
The back of each Deltora Quest book spoils the previous one. The summaries on the back of any book in the second and third series will tell you who the heir is.
PIGS DON'T FLY, the first book of a trilogy by Mary Brown, gives away the climactic surprise of the novel in the blurb on the front cover of the paperback original! Pigs don't fly.... "But dragons do," spoiling the secret that the winged piglet adopted by the heroine is actually a baby dragon.
In a simple case of Time Marches On, no contemporary reader of Sir Walter Scott's Ivanhoe is going to be fooled as to the identity of the mysterious Forest Ranger who calls himself "Locksley." Indeed, most readers probably won't even realize that this was meant to hide the fact that "Locksley" is Robin Hood in the first place! However, for readers in Scott's time, it wasn't a dead giveaway - Scott was the first author to use that word in relation to Robin, and though it's now (without exception) understood to be the birthplace of the famous outlaw, Scott used it as a pseudonym to initially hide Robin's true identity.
In Life of Pi, Richard Parker is a tiger. Though the fact that Pi will wind up on a boat with a tiger is never hidden, in reading Part One of the book, every reference to Richard Parker is phrased ambiguously enough that he could conceivably be human, and the reveal of his true nature as Pi is helping him onto the lifeboat is an Oh, Crap! moment. However, most descriptions of the story don't try and hide his identity as the tiger.
In the Rivers of London the climax of book one results in Lesley getting her face ripped off, the rest of the series she has to wear a face mask to cover the wounds. Since she is originally introduced as the World's Most Beautiful Woman, this has necessitated putting nearly everything she does in later books in spoiler tags.
The cover of City of Lost Souls spoils the romantic plot arc of the first three books of The Mortal Instruments, revealing that Clary and Jace are not brother and sister and, indeed, are in a romantic relationship.
Barely averted with Twilight Sparkle and the Crystal Heart Spell. This book was announced long before Twilight Sparkle was confirmed as becoming an alicorn, so the blurb was intentionally vague about what this 'Royal Event' actually was.
The penny dreadful-esque paperback re-releases of A Series of Unfortunate Events were clearly aimed at long-term fans rather than newer readers as the one-word subtitles were rather spoiler-y (i.e. The Reptile Room, or Murder!) The editions only got as far as the first three installments, but had the fourth been released it would have been under the title The Miserable Mill, or Hypnotism!, thereby giving away chapters' worth of intrigue in which Violet struggles to figure out why her brother Klaus is acting so weird. (To be fair, the back cover of the original did as much.)
Catching Fire, the second book in the The Hunger Games series, has "Against all odds, Katniss Everdeen has won the annual Hunger Games with fellow district tribute Peeta Mellark." as the first sentence on the back of the book.
The New York Times's obituary for Umberto Eco gleefully spoils just about every late-stage plot twist for The Name of the Rose imaginable with no forewarning, and to make it even more insufferable, describes the plot as if the reader hasn't read it yet.
The Kingdom Hearts II novel almost seems to be written to some degree as if for those who have already played Kingdom Hearts II, as well as some of the latter installments as well. It is much more open about certain things that in the game are not revealed until much later on or are obscured at best, such as that Riku is the hooded figure working DiZ and Naminé at the beginning of the game, that the Organization is using Sora/Roxas to collect hearts and contrary to what they think Nobodies actually can have hearts, something that is only made clear in Kingdom Hearts 3D: Dream Drop Distance. Saïx ponders over the fact that Axel has managed to gain a heart, and Naminé tells him point blank that he too has one, though he doesn't realize it.
It's revealed halfway through the first volume of The Unexplored Summon://Blood-Sign that the White Queen is the Big Bad and in love with Kyousuke. Since this is the main premise of the series, later volumes don't bother hiding it at all. Notable offenders are the covers of the fourth and fifth volumes, which depict the White Queen and Kyousuke together.
At the end of the fifth volume, Accelerator undergoes a Heel–Face Turn, but also suffers brain damage that leaves him dependent on external help to function. He then becomes the series' deuteragonist. The covers and blurbs of several later books don't hide this.
To an even greater extent, Othinus, the Big Bad for the first part of the second series. She succeeds in gaining her full power and destroys the entire world, then successfully breaks main character Touma by putting him through countless different hells. After he regains the will to fight, Touma talks her into a Heel–Face Turn and she restores the world to normal. Touma then fights the rest of the world to protect Othinus, who ends up losing all her power and turning into a tiny fairy who moves in with Touma as a second freeloader. The cover to the tenth book spoils everything except the last part, as it depicts Touma standing protectively in front of Othinus. There's also the twelfth book's cover, which depicts the shrunken Othinus on Touma's head.