Harry Turtledove's war-themed novels stress this element quite heavily. Many characters, including long-lived favorites, die, sometimes in completely random incidents. He seems to have a quota of "At least one death per book."
Rather than "Anyone CAN Die" it's more like "Everyone Probably WILL Die," particularly in the WWI books.
David Drake's military fiction (particularly his Hammer's Slammers) makes David Weber's Honor Harrington look like a piker. Only a handful of characters have relative immunity to this trope, though one apparently gets the axe, only for readers to later strongly suspect it was faked.
Unusually for children's books written in the 1960s, Lloyd Alexander had quite a few major, popular characters die in the final volumes of his The Chronicles of Prydain and Westmark series (but the protagonist and the leading female character were safe).
If you are a Bishōnen in a Dennis Cooper novel, you will most likely be kidnapped, raped, tortured, and/or murdered, and you will enjoy it. Especially if your name is George Miles.
How many people die over the main plot of an R. L. Stine book generally depends on the series—usually, none for main Goosebumps, one for main Fear Street or Fear Street Seniors, and anywhere from a couple to a massacre for any other side series. However, it's almost impossible to predict which books will kill a random (and potentially likeable) character at the end, which will Kill 'Em All, and which will leave everyone unscathed.
Dale Brown is not afraid to have characters who have lasted multiple books, like Brad Elliott, Wendy and Paul McLanahan, face the reaper. A Time for Patriots is pretty bad about this; while there are some fellows killed who are only introduced in this book, namely Leo and Ron, Jon Masters also gets killed off unceremoniously.
A notorious example is in The Stand where a number of prime protagonists die after almost a thousand pages of writing. Rumor has it that King himself didn't know where to take the story and realized only something this dramatic could kickstart it.
In The Acts of Caine, many central characters have died. Several have died and come back. One character got killed, came back as a semi-god, got killed again, and then became a true God.
Almost Night. Many characters die by the end of the book, and the rest of their fates are left unknown.
In the Aubrey-Maturin series, as the series nears the end of the historical timeline of the wars in The Hundred Days, some very major characters are killed off in essentially random and undramatic fashion: Diana and Mrs. Williams perish when Diana drives her coach too fast around a sharp corner, and Barret Bonden is killed by a long-range random shot from an Algerian galley.
John Birmingham's Axis of Time trilogy is full of these, specifically Dan Black, who dies between the second and third books not in combat but when a plane he's on crashes during take-off. Also Julia Duffy's best friend, who is shot in the head by a Japanese officer just to prove he is serious about killing civilians. The earliest examples include Captain Daytona Anderson and Sub-Lieutenant Maseo Miyazaki, who appear to be key characters, only to be found washed up on the beach in the next chapter. Both are beaten to death, while Anderson is also raped.
The BattleTech Expanded Universe novels, set in a setting where War Is Hell, frequently has main characters dying pointless deaths. And since it's set within a timespan of about a century, if assassination attempts, combat, or accidents fails to kill someone, old age will.
Glen Cook's gritty The Black Company has an appropriately gritty number of main characters drop off like flies from the titular mercenary group, occasionally brought back to life via deus ex machina so Cook can kill them in an even nastier way. It gets so that by the end of the series so far the company has been near-annihilated TWICE, and not a single character remains from the first book.
This is partially the point, seeing as a major message in the novels is that men may die but the Company lives on.
The world as it exists in Bone Street Rumba is one in which there are ghosts after death. So people can die the mundane way, and ghosts can die the deeper death, in which case they're gone from the world altogether as far as anyone knows.
In The Builders, specifically the fourth segment, several main characters are killed off in the span of just one or two chapters, and said chapters are only a few pages long. When the book ends, less than five named characters are still alive.
The characters in Tolkien's lesser-known novel The Children of Húrin fare far worse than those in The Lord of the Rings. By the end of the tale, Túrin (The Hero) is dead, in addition to his sister Niënor, mother Morwen, best friend Beleg, comrade Gwindor, rival Brandir, kinda-sorta love interest Finduilas, and a boatload of other minor characters. To add the tragedy, The Bad Guy Wins.
A. S. Byatt's The Children's Book takes its characters through WWI. All of the younger male characters enlist, and several of them die; the ones who survive do not return in the best of physical or mental health (and, to make matters worse, there are ominous rumblings of WWII ahead).
The Cold Moons starts with nearly 400 badgers. By the end, over 100 have died and only roughly 260 are left. All of the old Cadre were killed during the journey and the main badger antagonists all died as well.
The Chronicles of Thomas Covenant has more and more as the series goes on, until the Last Chronicles, where a so-called "major" character can be expected to die in nearly every fight scene. As the Second Chronicles prove, not even Covenant himself is off-limits.
In the Chung Kuo series by David Wingrove, leaders on both sides of the revolution have a tendency to die.
Cirque Du Freak by Darren Shan. Almost everyone dies, even Darren Shan himself, although he does reverse time and start over at the end.
In Dead Six, having a name just means the character is a target rather than collateral damage. Around half of the cast is killed in various gruesome ways, and very few of them are heroic deaths.
In Simon Green's Deathstalker series, after 1.2 million words, Owen is cut down in a simple street fight— and when he's dead they even steal his boots. He does get better in the sequel series, but a lot of other and protagonists don't.
Taylor Anderson's Destroyermen series has many of the original destroyermen die a few at a time each book. This also includes several important Lemurians, such as Nakja-Mur, the High Chief of Baalkpan, who's killed during the climactic battle in the third book. Each new ship, though, is named after a fallen main (or secondary) character. However, Word of God is that several characters, such as Captain Matthew Reddy and Chack Sab-At cannot die for plot reasons, and the fifth book opens with a quote from Courtney Bradford's book, which he is supposed to publish twelve years later, making his survival a Foregone Conclusion. The fifteenth and final book takes this Up to Eleven with many important characters dying, sometimes with barely a mention.
The Divergent trilogy kills off major characters in every book, and the last book Allegiant, kills off Tris the main character, which was shocking to the fandom. Allegiant is now regarded as one of the saddest YA books.
Each trilogy in the Dragons of Requiem series usually kills off at least two or three main characters. Sometimes nearly half the POV characters are dead before each trilogy ends, along with many Mauve Shirts.
The Dresden Files goes in and out with this trope. Most characters seem to be safe but occasionally a major character will be taken out to cement the noir nature of the series once more. Carmichael, a character who in most other stories would survive the entire series, gets shredded by a super-werewolf in book two. Morgan, Harry's Anti-Villain nemesis for a good portion of the series, gets killed off in a nasty way by a real villain.
As a real swerve Changes kills off Harry Dresden himself. The next book, Ghost Story, revolves around Harry solving that murder as a ghost. Though it then ends with him coming back to life.
Susan also dies in Changes, and Cold Days kills off Lily and Maeve.
Frank Herbert's Dune features the death of the entire cast through several generations. Some are killed off, some die of natural causes. Some even die more than once.
The original book features the death of Paul's first son by Sardaukar as well as a large portion of the Atreides household.
Dune Messiah is even worse with the death of Chani and seeming death of Paul after being blinded.
Children of Dune has Paul really die, completely broken and without his prescient vision, Alia also succumbs after becoming abomination.
Probably among the saddest is Leto II in God-Emperor of Dune. Spending 3,500 years as a worm hybrid, hating what he had become both physically and by his actions. He and his bride to be die just as he's found love in a way he's never experienced before. Because of how deep his transformation was, his death was also excrutiatingly painful.
The Bene Gesserit Mother Superior dies in both Heretics of Dune and Chapterhouse: Dune (Taraza and Odrade respectively). Miles Teg, shortly after being setup as a powerful super-being, also dies in the former.
In the last books, planet Arrakis has been annihilated and cannot support life any more.
In The Edge Chronicles it doesn't matter if you're a named character - you're probably going to die before the end of the book. Oh, you're still alive at the end of the book? Well here comes the prologue of the next book explaining how things have gone to hell, dumping you in a bad situation with events that're most likely going to result in your death in this book. Heck, even the main character of each sequence dies in the sequence after theirs!
The Enemy series by Charlie Higson pulls no punches and basically slaughters the cast in each book.
Fate of the Forty Sixth. Of the characters who survive, two are guaranteed to have gotten to the end by being in the present storyline. Every other character aside from Lieutenant Colonel Harris on the heroes' side dies.
FEED, by Mira Grant, is brutal with this. From beloved family members to main characters, nobody is safe. Which is as it should be in a world post zombie apocalypse, really.
Actually, GGK has a thing about leaving nobody alive. Someone major, often several someones, are killed off in almost every one of his books. Of especial note is Tigana, in which there simply IS no happy ending.
In The First Law trilogy, Joe Abercrombie makes it fairly clear early on anyone can die. In fact, by the end of the series the (initially) most identifiable main character winds up leaping off a cliff, whilst his band of followers have been slowly picked off across the trilogy.
The Gashlycrumb Tinies by Edward Gorey has this in an alphabetical form of 26 children meeting an each different fate.
Dan Abnett's Gaunt's Ghosts series of novels, set in the Warhammer 40,000 universe, are filled with plenty of fallen heroes. While the first few novels in the series don't feature many important character deaths beyond a few named soldiers and minor officers, by the later books the major Ghosts are being killed left and right as fast as new characters are introduced. Abnett proudly refers to himself as an "equal fatalities employer."
Similarly, by the end of Abnett's Eisenhorn series for WH40k, the eponymous character is the only one that survives all the way from the start. The rest are all dead or severely incapacitated.
The Warhammer Fantasy Battles novel Inheritance ends up with every single character seen in person in the first half of the book dead or, in the case of minor side-characters, probably dead. Although two of them are still undead, as opposed to dead-dead. Both of them having started out alive.
The Word Bearers trilogy does this remarkably well. While most of the main characters amongst the Word Bearers themselves are safe until the very end of book 3, every book manages to introduce characters, fleshed-out characters, and then at one point just kills them and never goes back at them. Of note is the character Varnus, an ordinary enforcer in the middle of the invasion. Roughly a third of the book is seen from his point of view, and then as an establishing moment for the series, he is killed off while one would expect him to join the main character. Aside from that (and this being 40k), there are many point of view switches with characters that you just know will die.
The GONE series by Michael Grant is a excellent example. It had around a 40% mortality rate, and many beloved characters died, including two cuties.
Joel Rosenberg's Guardians of the Flame novels abide by this, including a major character dying within the first fifty pages of the first book, the all-time fan favorite secondary character dying in the third, and the central character dying horribly in the fourth book. Justified, though, the world may seem like a simple D&D pastiche, but every decision made has real consequences, people choose wrong on a regular basis, and no one is sanctified.
In the Warhammer 40,000Grey Knights novels, survival is not guaranteed even if you've been part of the cast since book one, as Haulvarn proves.
The fourth, fifth and sixth Harry Potter books each ended with an increasingly major character dying. Then along came the seventh, which was a "bloodbath of epic proportions." It was so bad that Muggle Net took bets on character deaths before it even came out. Who died? Dobby, Hedwig, Mad-Eye Moody, Rufus Scrimgeour, Tonks, Remus Lupin, Fred Weasley, Colin Creevey, Peter Pettigrew, Ted Tonks, Bathilda Bagshot, Severus Snape, Vincent Crabbe, Bellatrix Lestrange, Lord Voldemort, 50 unnamed people, and oh, yeah, Harry Potter himself (sort of). And those were just the major characters. The complete list can be found here.
Mercedes Lackey, author of several series of novels, most notably, the Heralds of Valdemar series, makes use of this trope. It is lampshaded several times throughout the series, with characters noting that it's rare for the titular heralds to die of old age, as they most often die in service to king(or queen) and country. Also, given that the series lasts for over two thousand years (from The Black Gryphon to Owlknight), anyone who doesn't die in action will die of old age anyway.
His Dark Materials displays this trope more and more in each book, to the point where characters start dropping like flies in the third book the second they have finished furthering whatever minor plot points they had to serve.
David Weber's Honor Harrington series doesn't kill off memorable characters very often, but it is always a possibility. This can extend to characters who were present for several books of the long, ongoing series, such as Alistair McKeon, and to a lesser extent Jamie Candless. It can also extend to ongoing characters from the other side, whom Weber has gone to considerable pain to make you like — just before killing them off in brutal and heartbreaking fashion.
The Author's Note in the beginning of Storm from the Shadows explicitly states that Weber planned to kill Honor off at the end of At All Costs and restart the series with her children as the main characters. Fortunately, the series plot has advanced faster than planned, and now they won't be old enough during the upcoming action.
Weber's said the only character that's really safe is Honor's steward MacGuinness because his wife is fond of the character.
From one interview, when asked about the subject:
"Military fiction in which only bad peoplethe ones the readers want to diedie and the heroes don't suffer agonizing personal losses isn't military fiction: it's military pornography. Someone who write [sic] military fiction has a responsibility to show the human cost, particular [sic] because so few of his readers may have any personal experience with that cost.
Apart from the title character (who retires to his country estate after rising to the rank of Admiral), nobody in Horatio Hornblower is safe. Forester is a fan of showing that War Is Hell and also inverting Death Is Dramatic, so a number of important and likable characters die entirely off-screen. Several of Hornblower's proteges die horribly and his best friend Bush, who accompanies him through most of the series, is killed in an explosion with absolutely no foreshadowing whatsoever.
The Hunger Games trilogy plays this one hard, particularly in the final installment. Amidst the deaths of several supporting characters, Katniss has to witness firsthand the death of her younger sister Prim, whom she was trying to protect by entering the Games in the first place.
In Then by Morris Gleitzman, the sequel to Once, Zelda, one of the main protagonists of the series, is killed by the Nazis near the end of the book. She sort of reappears in Now but it's actually the granddaughter of Felix, the other main protagonist.
The Invisible Line series. Hell, in the first novella, both the protagonist and antagonist end up dead. They do end up getting better, but only under extremely rare circumstances.
The Kate Shugak series does not shy away from killing off regular and recurring characters, either through natural causes (e.g. Ekaterina, Old Sam) or murder (e.g. Jack Morgan, Mac Devlin).
The Last Black Cat, by Eugene Trivizas, is a story about how the humans hunted down black cats because they were convinced they brought bad luck, told by the point of view of the titular black cat. By the end of the book, only four of the many (named) cats are alive, with several off-screen deaths. And there are no 'nice' deaths in this book.
The norm in the Left Behind series, where cast members are constantly dying and replaced. And to rub salt in the many wounds, most of them die completely random and pointless deaths. By the climax of the series, not one of the original cast introduced in book one is still alive. But they all got better in the end.
Legacy of the Aldenata: Few of the characters from the first book with any development at all survive to the current book of the series, and sometimes they die or are believed to be dead several times.
While the Lensman series is mostly archetypical pulp sci-fi in which the heroes are ultimately able to overcome any obstacle, the first half of Triplanetary, the first book, is surprisingly full of this. Covering several time periods before the "present" of the rest of the series, it's not yet time for the apparently absent Precursor's plan to go forward so each part chronicles the sabotage of civilisation by the main antagonist and the deaths of both the protagonists and pretty much all other named characters. Unusually for a plan involving a Super Breeding Program, it's specifically important that the apparent romantic leads in each case die without ever actually becoming romantically involved.
Les Misérables. Both the main protagonist and antagonist are killed, along with almost everyone else.
Proven rather quickly in Limit (not the Frank Schätzing book), where almost all of the cast die in a bus crash within the first two chapters.
And Newt dies in the first chapter of the sequel, The Streets of Laredo.
Subverted in The Lord of the Rings: Gandalf, the second-most important character in the story, bites it halfway through the first volume, which is a huge caesura in the plot. Next, Boromir, another member of the Fellowship, dies in the first chapter of the second volume. Then, however, Gandalf comes back halfway through the second volume, and in the end the Fellowship and the other major characters on the good side — though they may experience various life-threatening situations, and side characters drop right and left — come out of a cataclysmic world war pretty unscathed; only old guys like Theóden and Denethor die. Interestingly, Tolkien at one time considered having Pippin and/or Sam die, as well as letting the Witch-King kill Éowyn, but he never had the heart to make it real.
A Lullaby Sinister. Main character status means nothing. If you notice that someone is inside the Surrogate School, chances are high that they will be killed in an extremely brutal way regardless of how prominent they are.
No one is safe in the The Maze Runner series, not even the main characters that the readers would expect have Plot Armor to go with. The first book kills off the Tagalong Kid and the youngest Glader, Chuck, and the third book — The Death Cure kills off one of the main sidekicks, Newt, and a Love Interest, Teresa. Then there's the first prequel, The Kill Order, which sees the deaths of the entire main cast (Mark, Alec, Trina, and Lana), with the only surviving major character (Deedee/Teresa) being only introduced 2/3 way to the novel.
In Mistborn: The Original Trilogy, Brandon Sanderson has no qualms about killing plenty of unnamed commoners and noblemen, the occasional minor character, and at least one main character per book. He's killed off quite a few Mauve Shirts in The Stormlight Archive as well, and has hinted repeatedly that the main characters are not guaranteed to survive the series, and given that the end of the first book sets up an impending conflict between several main characters....
At this point in the series, either book 4 or book 5 are going to focus on Eshonai. Eshonai is dead. From what we've heard, it's likely that the main character in the main parts of the book will be Venli, switching to Eshonai during the flashback sequences.
Edmond Barrett's Nameless War sees a mix of both character deaths and maimings.
Reality Ensues is in full effect in The Nexus Series. As of "Crux", Wats, Ted Prang, Mai, Warren Becker, Ilya, Dr. Holtzman, Jake, and Shiva are all dead, with things only to get worse with the looming war between humans and posthumans.
The rare romantic novel to embrace this trope, One Day kills off one of its protagonists about 2/3rds of the way through the book, completely changing the entire story.
Anne Tyler's The Amateur Marriage, which also revisits the two main characters at intervals throughout their lives, also does this - as well as the protagonists getting a divorce about halfway through the book. However, the marriage has changed the course of the surviving partner's life, so the rest of the book deals with that.
In the Parker novels, Parker himself obviously always makes it out. The series is not shy about bumping off anybody else, though.
In Fiona McIntosh's trilogy Percheron, being a named character is little protection. Along with a host of minor character deaths, only a couple of main characters survive the series.
Perry Rhodan had up to 23 immortality devices preventing aging and disease, but people could still be killed. Between issues 1399 and 1504(out of over 2500), the number of immortals went from 17 to 10. In the aftermath of that, 6 new devices were given to new holders. The 10 old immortals still live (some had near death experiences, one was repeatedly killed and revived), while of the 6 new ones 2 aliens laid them down due to not needing anymore, 3 humans were killed before their normal life expectancy was up, and one female alien was almost tortured to death by an insane space pirate, remaining sane only due to sheer willpower. Even ascended beings are not safe.
In Pretty Little Liars, most of the people suspected to be A (all of which were main characters) end up dying and maybe 2 of the people who actually were A. By the end of the last book in the series, the dead include Ali (maybe), Toby and Jenna Cavanaugh, Mona Vanderwaal, Ian, and Courtney.
The Prince Roger series has several characters that are upgraded to Mauve Shirt in the first book and promptly killed at the beginning of the second.
Project NRI. Everyone would very much like to leave Niege Research Institute, thus everyone would pretty much have to kill someone, anyone at some point.
Owlfred flew out of the room. She was in a place very far away from home, in a meeting room filled with potential murderers, including herself. Her name was Noriko Yamagi. She was 19 years old, and she wanted to get out of here.
Raised by Wolves has several examples, including the main character's primary love interest.
In the Red Mars Trilogy, the series ends with only two or three of the characters still alive. The main protagonist of Red Mars was killed off, The Lancer was killed off, and then everyone slowly started to die of old age. By the time of the later stories in the The Martians story collection, all the characters are dead.
The Redwall series has it lose several Mauve Shirts and at least one major character in every book. When asked about this, Jacques responded with "that's life".
Apparently, a body count of six-billion-plus in book one wasn't enough for Remnants - characters continue to die in every book following. By the series' end, fewer than ten Mayflower passengers were still alive.
In Alastair Reynolds' Revelation Space trilogy, the main character of Revelation Space is killed off in the second book, and pretty much everyone else introduced in the series prior to the last book dies. Only two of the characters survives the trilogy.
The Reynard Cycle: Each book in the series has a fairly high body count, and anyone other than Reynard himself seems to be fair game. Being a Mauve Shirt or a Tagalong Kid seems to be an almost certain death sentence.
Gods help you if you are a horse.
As a mystery series, Sano Ichiro has had regular and semi-regular characters be among the casualties and culprits of the mysteries, or be victims of their own failed plots in the shogun's Decadent Court. Ones that grow quickly in power often get killed off to symbolize how fast they can lose the shogun's favor, or when fate cruelly intervenes to show even the most powerful become prey to death. Seppuku, as a common way to regain honor for a shamed samurai, is also a common death.
Scarecrow by Matthew Reilly. Just prior to the climax of the book, Gant, the main character's love interest that has been part of the team for three books is suddenly and gruesomely killed off. Not to mention that 90% of the cast in each and every one of his books dies.
He also does this when Wizard is killed quite suddenly in Five Greatest Warriors.
Just don't get attached to a character in his books. Ever.
A Series of Unfortunate Events: Pretty much everyone dies, usually in horrible ways. The narrator is pretty vague about the fate of the orphans and their lost friends, pretty much only hinting that Sunny and Violet survive in obscure areas, the end and in The Beatrice Letters.
In Seven Men of Gascony by R. F. Delderfield, everyone died except the soldier Gabriel and the camp-follower Nicholette whom he marries and retires into civilian life.
Shannara loves this. Even if you survive your original series you will die in the sequel.
In The Silmarillion most of the main characters get killed at various stages during the war against Morgoth. Fëanor, the greatest Noldo (Deep-Elf) who ever lived dies in the first battle against the Balrogs (after accidentally killing one of his own sons in a fire, according to a very late story published in "The Peoples of Middle-Earth"). Of his half-brothers and nephews, who are the main protagonists of the Exile, the only two to survive are Finarfin, who regrets the campaign and returns back to Aman, and his daughter Galadriel, the future ruler of the Galadhrim. Only one out of the seven Sons of Fëanor might have been able to stay alive by the end of the First Age (his fate is actually unknown).
And those are elves. The Men die like flies.
The second volume of A Simple Survey, A Simple Monitoring. The first half consists of short stories, all but one of which include at least one death (and the sole exception has the protagonist murdering someone in the backstory). In some stories, only one person is left alive by the end. The protagonist(s) of each story aren't exempt from this.
Not quite literally so in the Slingshot series: while one of the main characters dies early on, and his death would have been avoidable, for the rest of the cast, the plot armor works pretty well.
A Song of Ice and Fire plays this trope to the point of the many main character deaths having become an internet meme - contrasting JK Rowling's quote, that "It's hard killing off so many characters" with a picture of George R. R. Martin, responding "You're adorable." Who initially seems to be the main hero doesn't even survive the first book. Parts of his family, their pets, their friends, their extended family, even some of their enemies, as well as beloved main characters (and their fairly fleshed-out rivals) from different story arcs bite it within the first book. Valar morghulis.
Martin explains in this interview that he employs this trope because he believes that if you're going to write about war you should treat war honestly.
Spectral Shadows has this. In fact all three of the main characters, Christine, Sir Jon and Rael, either have already died, or will die. And then there's plenty of death when it comes to the secondary characters as well.
The Star Wars Expanded Universe, more and more lately. There's a rule for the EU that Luke, Leia, and Han can't be killed. Everyone else is fair game.
In the X-Wing Series, a number of Rogues and other characters in the Stackpole books are lost, but since he never managed to get the reader to make an emotional investment there's not much impact. When Aaron Allston writes the Wraiths, each character is individual and interesting, and their deaths are more shocking and saddening. Jesmin Ackbar, Falynn Sandskimmer, Eurssk "Grinder" Tri'ag, Ton Phanan, Castin Donn.
There are some apparent deaths in Stackpole's mains, but they rapidly get better, usually by the end of the book (Looking at you, Lieutenant Horn).
Chewbacca's death in Vector Prime is the epitome of this trope in the Star Wars EU.
Characters first introduced in The Thrawn Trilogy are dying left and right. Zahn mentions that he's told that this is more realistic, and he admits that it is, but this is Star Wars, and he prefers entertaining to realistic. He's a bit higher on the "idealistic" side of the scale.
"While some authors (and readers) like the tension of wondering who will live and who will die, I prefer the tension of seeing how the heroes are going to think or work their ways out of each difficult or impossible situation they find themselves in."
In Emily St. John Mandel's Station Eleven, 99% of the human population dies in the Georgia Flu pandemic. Even 20 years later, characters can die suddenly and terribly.
In Strange Eons, almost all of the protagonists die and so do several other characters.
Don't get too attached to characters in Ian Irvine's Three Worlds cycle. Mauve Shirts on the verge of getting character upgrades? Fan favourite cameos? Plot-important characters? Main characters? It's one of the most brutal examples of Earn Your Happy Ending ever seen.
The Tomorrow Series. As if it wasn't bad enough that two of the main protagonists are comatose or dead by the third book, The Night Is For Hunting sees a raid on the group of children they've been keeping an eye out for; all but five of the children are killed, and one of those remaining dies of exposure not long afterwards.
It's not so much the number of deaths but the nature of each one. Corrie is shot in the back in Book 1, falls into a coma in Book 2, and dies at some unknown time between then and Book 6. Chris dies in a car accident offscreen. Robin goes out heroically, but her death may or may not have been pointless and demoralises the rest of them. And then there's the aforementioned children's deaths
Miles Cameron's The Traitor Son Cycle has characters who are well-established after several books be suddenly killed in a short sentence in a random battle, half the time they're not even mentioned afterwards.
Unda Vosari kills off at least two characters before the final chapter.
The Unknown Soldier. About half of the cast is killed, and most of the surviving characters are side characters. Out of the four most central main characters, only one survives the war alive - and he too is wounded and exits the story before the book ends.
Utopia 58: By the end of the story, every single major character, except for the villains and Kay's wife, is dead.
Probably an average of three cats, usually major characters, are guaranteed to die in any one volume of Erin Hunter's Warriors series.
However, this stops completely in Series 3, when except for one or two deaths of minor characters before the first book began, NO ONE DIES. Not even in Book 4, where despite containing the biggest battle since the First Series, NO ONE dies. They had two near-deaths. Then Book 5 came...
Counting the deaths seen in Jayfeather's visions, the third series only killed off 6 characters (one of them an unnamed elder) and a whole bunch of Tribe Cats, which is pretty minor considering the first two series each have body counts in the twenties. And of course, Hollyleaf might not actually be dead...
Counting unnamed characters, kits, deaths that are only mentioned and not seen, deaths by famine and sickness, and the four cats that were left behind to die in Dawn, the actual average number of deaths in the first two series is around 4.75 per book. (Until the Power of Three series, which brings the number down. But then of course, no one knows how many Tribe cats were killed in Outcast.)
And then there's Bluestar's Prophecy, a prequel with a large cast of characters, most of which are never seen in the first book. Guess what happens to them (although, a fair number of them did get killed off in between chapters).
So far, the fourth series seems to be working on some form of subversion of Sudden Sequel Death Syndrome: If you are a minor character who has been alive since the first series, you will be arbitrarily killed off without warning. Meanwhile, The Last Hope features multiple cases of The Hero Dies, and a total of 32 cats died in the Dark Forest Battle, making the fourth series the arc with the highest kill count.
The fifth series (the prequel arc) brings this back full force, killing off more than 15 characters in the first three books alone. The last three books generally have one major death per book, however.
The sixth arc has killed off several major characters so far, the death of Needletail being an especially jarring example. Not to mention, more than half of SkyClan died or defected in the backstory, major and minor characters alike.
The seventh arc seems to be toning down on this, with a kind-of death in Book 1 and one death in Book 2. However, in Book 3, a total of 10 cats were killed, supporting and minor characters alike.
The novella Mapleshade's Vengeance kills off 7 characters, including 3 kits and the titular character herself.
Watership Down is known for its atmosphere of pervasive dread, but the author turns out to be much too kind-hearted to pull the plug on his favorite characters, and settles for simple maiming instead. (I guess being rabbits, Kill 'Em All would be the default outcome, nothing to write home about.) See animated film version, however.
He had planned to kill off Bigwig at the end though, and only spared him at the behest of his young daughters. The mentality was definitely there.
Wearing the Cape begins with a terrorist attack that leaves bodies all over, the Sentinels are shown to have lost several members before the story begins, the murder of a street-level hero is casually alluded to, and finally, in the attack on Whittier Base no fewer than three Sentinels die—including two main characters.
In the final volume of The Wheel of Time, Robert Jordan killed off characters left and right, including, but not limited to, Siuan Sanche, Gareth Bryne, Davram Bashere, Rhuarc, Hurin, Alanna, Gawyn, Birgitte, and even Egwene. Also the Memetic Badass horse Bela, though that was unplanned and due to Brandon Sanderson writing her into a situation the editor thought she couldn't survive.
Worlds of Shadow: Many of the starting characters don't make it through to the end of the series.