Used extensively in the Age of Fire Series. The three separated siblings often her tales and legends of other's feats, factions from previous stories reappear and affect others, and some recurring characters are met by all 3 siblings.
Angela Nicely: In “Girls United!”, Angela says that Mrs. Nicely isn’t keen on football, since when Bertie played it, the ball broke her greenhouse window. This happened in the Dirty Bertie story “Smash!”.
Artemis Fowl gets a very nicely hidden one. For those who read second book enough, they'd realize that the 6th book revolves around something very briefly mentioned in the 2nd on page 120.
... "My father, though some of his ventures were undoubtedly illegal, was... is... a noble man. The idea of harming another creature would be repugnant to him." Holly tugged her boot from eight inches of snow. "So what happened to you?" Artemis's breath bloomed in icy clouds over his shoulder. "I...I made a mistake."
Artemis briefly mentioning the event that the 6th book's plot revolves around.
Without foreknowledge, the "mistake" he was referring to could just as easily refer to him abducting Holly.
"The Bicentennial Man": Chapter 13 strongly implies that this story takes place in the same continuity as I, Robot, because the Robertson family name is still in charge of the corporation (although it is now Smythe-Robertson), and there's a hologram of Susan Calvin on the wall, a reminder to everyone at US Robotics of the corporation's first Chief Robopsychologist.
"The Imaginary": Because this story was printed in a different magazine compared to "Homo Sol", it was important to reference Tan Porus's work to convince the humans of Earth to join the galactic council.
The Robots of Dawn: This novel takes place in the far future where robots are common in the Outer Worlds (they actually outnumber humans). While introducing the plot, Han Falstofe makes a reference to one of Asimov's short story from I, Robot, "Liar! (1941)", which takes place in the early 21th century, about the time robots got developed in Asimov's universe. The story is told by Falstofe as a legend which is most probably not true, but illustrates the "mental freeze-out" of robots.
Lawrence Block's Bernie Rhodenbarr series:
In The Burglar Who Painted Like Mondrian Bernie does some running, which is a reference to his claiming to be a runner during a podiatrist's appointment in The Burglar Who Studied Spinoza. His new lawyer, Wally Hemphill, was a minor character in the same book.
In The Burglar Who Traded Ted Williams Bernie briefly mentions the bodies found in Burglars Can't Be Choosers and The Burglar Who Painted Like Mondrian.
This is done again in The Brothers Cabal when Cabal mentions the fate of the Skirtingboard people-which were also featured in the Blustery Day.
At the very end of A Canticle for Leibowitz, the abbot finds the skull of Francis, the protagonist of the first third of the book. It is, of course, symbolic: both men complete their mission and ensure that humanity can grow and flourish once more, but neither survives to witness the consequences.
Michael Connelly has been writing mystery novels since the early 1990s and they are all in the same universe, and all of them after the first have at least one continuity nod. Here are two examples: 9 Dragons starts with Detective Harry Bosch investigating the murder of an Asian Store-Owner—at the same store where Bosch wound up at the end of a previous Connelly novel, Angels Flight; non-Bosch novel The Poet is tied to the Bosch verse when a character reads a newspaper article by Keisha Russell, a character mentioned in Bosch book The Last Coyote.
The Cosmere has numerous subtle ones tying all the works in it together:
Wax And Wayne also has numerous nods to the events of Mistborn. Various locations are named after prominent characters from Mistborn, other characters have become mythic figures during the Time Skip, Wax is a distant descendant of Breeze, Spook’s nigh-incomprehensible slang has become an officially recognized language, “Hazekillers” is now used as slang for anti-allomancer weapons, and techniques invented by the protagonists of Mistborn have become widespread.
Mistborn: Secret History has Kelsier meeting a ton of Worldhoppers from previous stories, with appropriate references to said stories, including Khriss and Nazh and some Elantrians. He also runs into Hoid again, who references their previous meeting in The Original Trilogy and chews Kelsier out for some of his actions in that story.
White Sand has a young, pre-worldhopping Khriss show up, setting up her chronologically later appearances in the other books.
In Sixth of the Dusk, it’s noted that the planet the story happens on, First of the Sun, has a naturally occurring Perpendicularity, a place that allows for dimensional travel that figures into the plot of some other Cosmere stories and plays a major part in the backstory.
The Ghostblood’s trophies are a sort of “greatest hits” collection of objects, locations, and types of magic from past stories; a vial of pale sand from Taldain, Hemalurgic spikes, a lock of hair stolen from a Returned, a Tear Of Edgli, a silver knife from Threnody, etc..
It's not uncommon for stories in the Cthulhu Mythos to make reference to one another in some way with varying degrees of subtlety. When it's not bringing in one of Lovecraft's monsters, this can range from using common elements (i.e. the Necronomicon, referred to frequently by Lovecraft and used just as often by other writers) to referencing the events of specific stories.
Daystar has several references to the earlier books in the series, including a passing mention of Firebird wearing a "gold bird pendant" which is presumably the eagle pendant Brennen's older brother gave him when they were both children and which Brennen had carried with him for years, and the fact that Tel Tellai married Esme Rogonin, whose friendship we see beginning in Crown of Fire.
In Double Down, Greg says that the Underpants Bandits books (from The Long Haul) are old news now and that Spineticklers has replaced them.
Wrecking Ball:
A picture on Rowley's collage depicts him and Greg in their two-headed monster costume from Double Down.
Various characters from The Meltdown appear attending Greg's going away party.
There are three references to Diary of an Awesome Friendly Kid. Two illustrations (Greg's baby pictures and Rowley with an ice cream cone on his head) from said book reappear here in Greg's style, and a picture that Rowley draws in cement is done in the Awesome Friendly Kid style.
The Family Frolic magazine from The Long Haul and Double Down appears once again, with Susan using it to help Greg with the yard sale.
In The Deep End, Sweetie is mentioned once again. And he's still as spoiled and overfed as ever.
In Big Shot, Greg's team is initially sponsored by Marconi's Sub Shop, which appears in The Meltdown. Greg is suspicious because they've been closed due to health code violations. Later, Coach Patel takes them there to eat at the end of the season.
The Divine Comedy: At the end of Paradiso Canto 22, Dante looks back at the seven spheres he passed and notes that back in the second canto, he incorrectly argued that the sphere of the Moon must be made of rare and dense matter.
Discworld has quite a few. One of the bigger ones is a scene in Making Money where Moist is told "You can walk out of that door over there, and the matter will not be raised again." As the last time this happened, the door opened onto a very deep pit, he is a little wary. It is, in fact, a normal door.
This crosses through different protagonists' books. In Thud!! Vimes is complaining about "that pea-brained idiot at the post office" (Stanley Howler, Head of Stamps as of the end of Going Postal), whose cabbage-scented stamps apparently ran into a few unforeseen problems, and in Making Money Moist briefly apologizes to Vetinari about the cabbage stamp debacle when trying to think of reasons Vetinari might have wanted to meet with him.
In Night Watch Discworld, the entire plot is set in motion by lightning striking a particular house as Vimes falls the University library roof straight into L-Space. This doesn't seem especially meaningful, unless you've read Thief of Time, and know that lighting bolt stopped time and 'eventually' shattered time into a billion pieces, with Vimes apparently being missed (probably due to him being inside L-Space) as it got put back together.
A scene in Thief of Time nods 'back' in reply, as characters from TOT notice there's a fight going on in Sator Square that is the aftermath of the action at the very start of Night Watch Discworld. Which makes sense as we're observing it about 5 seconds later.
Also, in The Truth Vetinari asks William if Dibbler had had any managerial position of the press. This makes sense when you realize that Dibbler has almost destroyed Ankh-Morpork several times this way: In Reaper Man he had the snow globes, in Moving Pictures there was the "clicks", then in Soul Music there was Music with Rocks In...
The Dark Morris Dance from Reaper Man later appeared in Wintersmith as a key plot point. Also, Wintersmith ends with Rob Anybody of the Nac Mac Feegles struggling to read "Where's My Cow", Sam Vimes Jr's favorite book from Thud!.
In The Grass Widow's Tale, George Felse stops in at a garage/petrol station he visited as part of the investigation in Flight of a Witch, four novels and four years earlier, and reflects on how things have changed in the intervening years.
Rainbow's End is set in the same neighbourhood as The Knocker on Death's Door, three books previous, with some returning characters in supporting roles.
Foundation (1951): In The Merchant Princes, Sef Sermak, the opposition leader from The Mayors, is mentioned as a historical figure who broke the power of the aristocracy in the Four Kingdoms with his land reforms. His party, the Actionist Party, is now in government.
From the chapter "The Prince's Tale": "Keep an eye on Quirrell, won't you?"
There are about a million of 'em in that chapter as Harry goes through all of Snape's relevant memories—Dumbledore and Snape's conversation in the woods (where Snape says "you take too much for granted") is the one Hagrid overheard, as reported in HBP, we hear "that awful boy" (Snape, not James) telling Lily about Dementors, as overheard by Petunia and reported in the beginning of OotP, etc, etc...
Book Five is basically one Continuity Nod after another. When Harry takes his exams, he reminisces over all the stuff he did and learned over the past five years.
The Horatio Hornblower novels have more than one nod to events that came before. The series began in the middle of Hornblower's career and the next several books go in a direct line of continuity, but then jump back to his days as a midshipman. Hornblower and the Atropos therefore seems to contain all the references to specific ships and adventures from Mr. Midshipman Hornblower and Lieutenant Hornblower that later books lack (while not mentioning his immediate previous command, Hotspur, which Forester wouldn't write for another decade).
Nick Hornby makes nods towards his other novels:
In About a Boy, Will shops in the record shop from High Fidelity.
In How to Be Good, one character lives in a flat in the same building as an employee of the same record shop.
In A Long Way Down, a character from How to Be Good has a show on Martin Sharp's terrible cable TV channel.
One of the protagonists of the Hostile Takeover (Swann) series is a Frankenstein whose ancestors were optimized for neural interface with computers. She shares a surname with a frankenstein hacker who plays a significant role in ''Specters of the Dawn''
Various James Bond novels have mentions of Bond's previous adventures. Most common references are his short marriage from On Her Majesty's Secret Service and his various run-ins with the criminal organization SPECTRE.
The Johannes Cabal series does this twice with minor throw-away supernatural weather terms brought up in the very first Cabal short story "Johannes Cabal and the Blustery Day" showing up first in the epilogue Johannes Cabal the Detective where we encounter the Blood-Red Snow and its creator, and then later in The Brothers Cabal when Horst encounters Cumulonemesis.
John le Carré occasionally adds references to names and events from earlier books that aren't directly relevant, just to reinforce the fact that his spy novels share the same setting.
The Looking Glass War centres around The Department, a different agency to George Smiley's employers the Circus. An early chapter very briefly mentions Smiley's Circus colleagues Leamas and Guillam, who play no further part in the story (unlike Smiley himself).
The Lord of the Rings, in addition to many more or less obscure allusions to events from The Silmarillion and The Hobbit, manages to do an odd little in-universe continuity nod as Sam realises that he is indeed part of the same continuity, which are legends to him.
"No, sir, of course not. Beren now, he never thought he was going to get that Silmaril from the Iron Crown in Thangorodrim, and yet he did, and that was a worse place and a blacker danger than ours. But that's a long tale, of course, and goes on past the happiness and into grief and beyond it - and the Silmaril went on and came to Eärendil. And why, sir, I never thought of that before! We've got - you've got some of the light of it in that star-glass that the Lady gave you! Why, to think of it, we're in the same tale still! It's going on. Don't the great tales never end?" (The Two Towers, "The Stairs of Cirith Ungol")
Gandalf getting dragged into a pit by a Balrog he has defeated can feel like one to "The Fall of Gondolin", where the Elf Glorfindel is dragged off a cliff by a Balrog.
An especially evil one in Balin's grave. While Tolkien is not big on torturing his readers, killing one of The Hobbit's most beloved characters in this way is plain cruel.
Towards the very beginning of Magician, the first book of The Riftwar Cycle, Pug is named squire of Forest Deep. In King of Foxes, the 20th book, an agent is told that if he needs to contact the Conclave of Shadows, he should address a message to the Squire of Forest Deep and give it to a certain person. This may not be a code phrase. Pug is the head of the Conclave, and when he renounced his title as Duke of Stardock, he did not renounce his other title, that of "Squire of Forest Deep".
David Mitchell’s books are noted for their interconnectivity. This is true within single stories (the wondering soul in one of Ghostwritten’s narratives, whose travels take it full-circle); within single novels (Ghostwritten and Cloud Atlas which are both made up of several independent but connected stories), and between novels (and other works). For example, a character from the Frobisher narrative in Cloud Atlas features prominently in Black Swan Green. A minor character from Marco's narrative in Ghostwritten starts his story by waking up to a woman whose birthmark marks her as an iteration of the 'soul' that links all of the narratives in Cloud Atlas. The list goes on and on. Even in Mitchell's latest book, The Thousand Autumns of Jacob de Zoet, which was seen as a departure from his previous meta/post-modernist fiction into fairly 'straight' historical drama, there is at least one very subtle connection to his earlier book Number 9 Dream: the minor character Satsuki Miyake comes from Yakushima, hinting that she is the ancestor of Eiji Miyake, protagonist of the earlier work, who also hails from the tiny island. Insofar as Mitchell is writing about the 'real world', past or contemporary, this Verse is quite close to our own. However, Mitchell is also notable for writing science fiction elements into his books. If, as seems to be the case, all Mitchell's works are taking place in the same Verse, we are left to try and reconcile the end of Ghostwritten (which implies the self-aware super-computer created by the nice Irish scientist has decided to annihilate mankind) with the future-set episodes of Cloud Atlas (in the first instance a Soylent-Green-referencing consumerist dystopia; in the second instance a far-future-set 'last days of humanity'). The possibilities are fascinating...
The Bone Clocks goes even further and connects almost all of his previous novels and fleshes out the entire multiverse. In particular, The Thousand Autumns of Jacob De Zoet is directly connected with Dr. Marinus revealed as an Eternal Hero and Enomoto's immortality cult as legitimate magic. The future setting of Cloud Atlas and some background on the Prescients are also tied into it.
Nnedi Okorafor is fond of doing this, using references to her made-up world Ginen to loosely tie together Zahrah the Windseeker (set on Ginen), The Shadow Speaker (set partly on Ginen), Who Fears Death (the heroine finds a book about Ginen), and Akata Witch (the heroine's knife was probably made on Ginen).
In “What’s Your Talent, Roys Bedoys?”, Roys makes the same cupcakes he made in “It’s the Last Day of School, Roys Bedoys!” (putting a scoop of ice cream on top).
In “It’s St. Patrick’s Day, Roys Bedoys!”, Maker wears his green shirt with the white stripes he wore back in “It’s Spirit Week, Roys Bedoys!”.
In “Roys Bedoys and Little Red Riding Hood”, the kids acknowledge that they’ve put on a play before (back in “Roys Bedoys and the Three Little Pigs”).
In “Don’t Exclude Your Friend, Roys Bedoys!”, Roys and Maker remember when they went camping and Flora told a story which made Wen laugh, which happened in “Let’s Go Camping, Roys Bedoys!”.
“Christmas is the Season of Giving, Roys Bedoys!” reveals that the Bedoys boys eventually managed to get the video game that sold out in “It’s Black Friday, Roys Bedoys”.
The helmet from “Don’t Exclude Your Friend, Roys Bedoys!” appears again in “Manage Your Anger, Roys Bedoys!”.
In “Read a Book, Roys Bedoys!”, Flora reads The Hat on the Cat, which is the same book from “Behave at the Library, Roys Bedoys!”.
In John Mortimer's Rumpole of the Bailey stories, Rumpole (the first person narrator of all but a few of the stories) often makes reference to other cases he's worked on (with Mortimer going so far as to add footnotes referencing the title of the story, the title of the compilation volume in which it appeared, and the page number it started on).
The Rumpole series is also noteworthy for perpetrating what may well be the greatest Noodle Incident / Continuity Nod transformation in the history of literature. For years, in just about all of the stories Rumpole would reference his greatest case, "The Penge Bungalow Murders" (which, as he often noted, he tried "alone and without a leader"). Finally, nearing the end of his writing career, Mortimer finally wrote the novel Rumpole and the Penge Bungalow Murders, neatly turning that Noodle Incident into a Continuity Nod.
Pelevin's The Sacred Book of the Werewolf contains several references to his earlier "A Werewolf Problem in Central Russia", and implies that Alexander in The Sacred Book of the Werewolf and Sasha in "A Werewolf Problem in Central Russia" are the same person.
In Shaman Blues, Dora Wilk shaking up the entire power structure of the Council at the end of Dora Wilk Series is why Gardiasz is going full Da Chief on Witkacy.
In the Sherlock Holmes stories, Dr. Watson was fond of making references to other cases Holmes had worked on. While many of these were to incidents that Conan Doyle never based stories on, a few were references to other stories found in the Sherlockian Canon.
Jon Snow's advice to Arya in book 1 of A Song of Ice and Fire to "stick them with the pointy end" of her sword is referred to by both of them throughout the series.
During the books, Tyrion loses his nose. Much later, when the dwarf Bobono, who plays Tyrion in a play, starts groping Mercy( Arya),she threatens to rip off his nose if he doesn't stop.
Peter David's Star Trek: New Frontier novels include an immortal character named Morgan Primus. It's hinted that the character may be the same as Number One (from "The Cage"), suggested that she is the original model for shipboard computers' voices, and both Jean-Luc Picard and Montgomery Scott mistake her for other women they're familiar with (Lwaxana Troi and Christine Chapel, respectively). All of which is a big wink at the numerous roles that actress Majel Barrett (-Roddenberry) has filled in the Trek Verse.
In the Rihannsu novel The Empty Chair, it’s mentioned that there’s a planet just barely inside the Rihannsu side of the Neutral Zone that is almost richer in dilithium than Direidi. In Swordhunt, a Klingon refers to a "Thought Admiral".
Tons and tons of them in the modern novel line. Just one example: in Star Trek: A Time to Kill, President Zife ends up getting a list of crises which occurred on his watch recited to him. These are, of course, all references to other novels. There was the Genesis Wave, the Holostrike, the Trill debacle, the Selelvians...
The Fortunes of War books are set in the later years of the TOS timeline, and Battlestations! makes reference to this by having Kirk own a racing yacht named Edith Keeler. Piper, the main character, ponders the meaning behind the name.
In Dark Force Rising, Luke and Mara sneak aboard a Star Destroyer to rescue Talon Karrde. Luke sneaks into and out of the detention area via a trash compactor. (Also, earlier, looking to disguise themselves for the rescue attempt, Luke advises Mara that stormtrooper armor is hard to see out of, referencing his line from A New Hope.)
Luke: I can't see a thing in this helmet!
References to "I have a bad feeling about this" are rife in the Legends. And Death Star is absolutely packed with acknowledgments about the builders of the Death Star, the prototypes, and what else is going on in the galaxy.
Star Wars: Legacy had an "I love you." ... "I know." exchange between two characters, one of whom is a descendant of Han and Leia.
The Legends continuity is unusually continuous, for an Expanded Universe. In theory all books are a view of the same 'verse; some views are crystal clear, some are blurry, and some are downright abstract. In practice it depends on the writer. Karen Traviss does no research at all and merrily tramples on previous canon, then calls people who dislike this the "Talifan". Timothy Zahn reads everything, takes it into account, and then incorporates details and characters freely and subtly, though he retcons things here and there. In the first book of the Hand of Thrawn duology, he introduced the Caamaasi species, though none of them were named characters. Immediately after that, Stackpole wrote "I, Jedi" which had a Caamaasi character, and in the second book of the duology Zahn included this character in a fairly prominent role. The two books of the duology were published a year apart. The character was in character. Most writers lean more towards Zahn than towards Traviss, but is it any wonder who is more loved?
While authors try to mimic Zahn in this way, LucasArts itself seems to merely despise the fanbase when they ignore the idea that there is any pre-established continuity. They ultimately decide the outline of new Star Wars material, and in an actual inversion of the Continuity Nod that goes beyond simple Retcon, they add new events to the timeline thousands of years apart and then connect them together. For example, the new Fate of the Jedi series, which takes place about 40 years after the movies, involves plot elements from 5,000 years before movies. Previously, the span of important events covered to about 4,000 years before. When they ran out of room to reference events, they just tacked it on the end, and at an arbitrary date no less. Plans spawned from said 5,000 year plot must coexist alongside other plans, such as Palpatine's rise to power, The Sith Empire from The Old Republic hiding in the Unknown Regions, and The One Sith (which is itself another example of this): all of these plans manipulate politics and events at a galactic level, yet they never contradict one another. This new element is simply the newest of a long trend, and one that just ramps it up to eleven. The characters are unaware of these plots (as is the audience) but the Galaxy Far, Far Away, in its omniscience, knew about them the whole time.
Star Wars: Kenobi, taking place just after Revenge of the Sith, references events from the prequel films and Star Wars: The Clone Wars, as well as previous literature and comics, both of events that took place on Tatooine and of wars from thousands of years in the past.
Early in the first book, Leia mentions all the villains from the previous books, and it had just been one after another there for years.
At the end of the third book, the computer-controlled Imperial fleet automatically sets course for Byss, the Emperor's secret secondary capital in the Deep Core from Dark Empire.
Tantei Team KZ Jiken Note and its spinoffs make nods among its own novels or between each sub-franchise, in different ways:
Within KZ itself, any continuity nods will explicitly state which previous book of the franchise that reference happened in, for example:
I first went to Wakatake's house during The Egg Hamburg Steak Knows''.
KZ and KZ Deep File are taken to be of the same universe; so plot points do cross-reference. Novel titles were no longer cited, however. For example, the opening lines of Cherry Hills Hold a Crime mentioned the fact that Uesugi had a brief drop in grades. This, refers to KZ's The Backyard Knows, one of the four novels that gets adopted into anime.
In The Immortals, the second quartet in the universe, Daine meets Alanna and the other major characters of Song of the Lioness in Wild Magic. She spends the next two books doing other things with her teacher Numair, but they come back in the fourth when war breaks out.
In the third book of Protector of the Small, King Jon's royal progress makes stops at the fiefs of Eldorne, Tirragen, Malven and Sinthya—who are all "invited" to host the royal retinue at expensive banquets. This is mentioned as a tactic of Jon's to remind them he's keep an eye on them after their treasonous actions in the first two quartets, and to drain their pocketbooks so they can't try rebelling a second time.
H. Beam Piper example: in Uller Uprising, there's a romance between General Carlos von Schlichten and his adjutant, Paula Quinton. In Federation, the short story "Oomphel in the Sky" has a reference to a Paula von Schlichten Fellowship.
In Isabel Allende's Violeta, Violeta Del Valle mentions that her paternal grandmother, Nívea, was decapitated in a strange accident. She also mentions his sister. Just from the nameless aunt’s description as clairvoyant, it’s clear she talks about Clara from
In Cetaganda, Miles was on the titular planet to attend a state funeral, and was talking privately with one of the keepers of the Star Creche, haut-lady Rian. The discussion is interrupted by a call to Rian from a Cetagandan agent, ghem-Colonel Millisor about tracking down some useful genetic line. Millisor was the antagonist of an earlier novel, Ethan of Athos, which took place at the same time but was written ten years earlier. The plots have nothing to do with each other outside of this one call, though Miles does note that this will be a useful fact to distract Illyan with when he gets back to Barrayar and has to report. Presumably this led to the events of Ethan of Athos, which in turn led to the mission Miles gets sent on in the short story "Labyrinth", which introduced several major supporting characters.
Cetaganda results in him being given, in his words, "a lead weight, suitable for sinking small enemies." Namely, the Cetagandan Order of Merit, their third highest honour. Ivan asks if he'll ever wear it, and he says only if he ever needs to be really obnoxious. In Memory the need arises, and it is briefly commented on.
Gaunt's Ghosts has quite a few nods to other books Dan Abnett has written, and almost all of his books include references to the events and characters in his other works. Many, however, are retroactive, having been written about first as a passing reference, then developed into a full story. This can result in circular Continuity Nods.
Dan Abnett's Horus Rising opens by recounting how Loken would say that he was there when Horus killed the emperor. This is, in fact, the emperor of a planet they conquered while he was still a loyal son of the Emperor, but it's not lacking in Irony.
Indeed, if you know the backstories well enough, the entireHorus Heresy series contains at least one example per chapter that is just like that. Fans will often yell 'No! No! Don't do that! You stupid plonker!' at certain actions, and frankly groan at the various lines that are just dragging the currently on-top-of-the-galaxy Horus deeper and deeper into wannabe Evil Overlord territory.
Also, note that the first chronological literary appearance of the Dies Irae was in the very first Heresy novel, and it went on to play a major part in the series. Dies Irae appears ten thousand years later as a Chaos Titan that played a major part in Graham McNeill's Storm of Iron novel, written long before the Heresy novels.
However, could be justified that this a Chaos titan, and that the power of Choaos has helped to maintain the titan all those years.
Another Heresy novel, Mechanicum, references a Chaos-corrupted Titan on Mars being destroyed, but whose hellstorm cannon was salvaged and taken to another world - presumably Kronus, from Dawn of War: Dark Crusade, which had a hellstorm cannon play a significant role in a stronghold siege and, if you were attacking as the Word Bearers, turn out to have a dormant Daemon trapped within.
One of the first Ciaphas Cain books makes reference to there being record of a Commissar who held the dual rank of Colonel-Commissar, a Shout-Out to Abnett's Ibram Gaunt.
Black Legion tells the tale of how Justaerin came to be corrupted and possessed, two qualities that are noted in Night Lords, written much earlier, but taking place millennia later. Its main character was also alluded to in John French's Ahriman novels.
In the Warrior Cats novel Sunset, Firestar says to Brambleclaw, "Remember when I had to go away for a while, when you were a new warrior?", and talks about how Graystripe said he'd wait for Firestar to return, as a reference to Firestar's Quest. Interestingly, Sunset came out over half a year before the release of Firestar's Quest, so it referenced a scene that fans didn't know about yet.
Used in-universe in Watership Down, in the legends of El-ahrairah. In the "Black Rabbit of Inle" tale, the folk hero's whiskers are gambled away, then re-grown at the end. Days later, as he's beginning the "Rowsby Woof" story, Dandelion mentions El-ahrairah's new whiskers.
Echo has a brief mention of a town in the Hudson valley that disappeared, implying that HEX (the international version, that is) takes places in the same universe.
In Young Royals, there's a small one that's easy to miss, but the author, Carolyn Meyer, also wrote Isabel: Jewel of Castilla from The Royal Diaries, which focused on Catherine's mother, Isabel. In Patience, Princess Catherine, Catherine recalls being told of how her parents first met and the details line up exactly with how it was described in Castilla, down to Isabel wearing the same lavender dress with a ruby necklace and exclaiming the same line upon seeing her husband-to-be ("It is he! Oh, it is he! And all that I could wish!").
