Books of "literary merit" often have a preface that discusses the meaning of the book, casually throwing major plot points out there.
Related are all these teachers who, when assigning their students novels for mandatory reading, casually spoil everything about the plot, because Lord forbid the students actually derive pleasure from reading.
Even worse than a spoiler summary on the back cover, there are a few books which back cover is an actual extract of the book. Not a matter when it comes from the beginning, but sometimes it comes from the ending. There is a French edition of Salammbô whose back cover is an extract of one of the last pages, describing the death of the main protagonist.
The Polish publishing house Amber seems to have a thing for horribly spoilerish blurbs. In an edition of Strugatsky's The Powerless of This World, the back cover blurb is only the surprise ending, and nothing else. A Polish edition of Robert Sheckley's Dimension Of Miracles likewise spoils the humorous ending, that the hero gets back to his world but finds it insufferable. And the one for A Stainless Steel Rat Is Born spoils the death of The Bishop, and even gets it completely wrong (claiming that he's killed by the police in an ambush, while in reality he's killed in a military attack on a distant quasi-medieval planet).
If you're going to read a novel published by the Penguin Classics line, just don't read the back cover blurb. The worst is the Penguin (and almost any other edition of) Little Women, which states plainly that readers will "cry over Beth's untimely death", which doesn't happen well into the second half of the novel. This might be a case of It Was His Sled, but still.
Craig Brown (British columnist) wrote an article deploring this practice, citing several egregious examples - one which sticks in the mind is a blurb along the following lines: "in this gripping narrative, the reader slowly realises that the narrator is insane" - thus preventing the reader from slowly realising anything of the sort...
The novels of Edward Rutherford (Sarum, London, etc) always include a family tree for the characters, which tracks them through the generations and centuries. Nice and handy ... except that it always spoils who survives to reproduce, who marries whom, and which families will attain noble titles. Could easily be averted if they put this at the back of the book, instead of next to the maps which you're always having to flip back and reference.
Ed McBain's 87th Precinct novel Mischief: the book cover for at least one Italian edition reveals the secret plan of the Deaf Man - what he wants to steal, from who and when. The only problem is... the plan is actually discovered only at the very end of the book!
One edition of Animal Farm has one of the biggest twists spoiled right on the front cover: the front cover proudly displays the quote, "All animals are equal, but some are more equal than others". For some, this is a case of It Was His Sled, as this book is commonly assigned to high school students, but for students who weren't assigned it in class and wanted to read it for gasp fun, the book is completely ruined.
The Harper Collins paperback 2000 version of the Aubrey-Maturin series every book has a summary of between 2 and 4 of the next in the series (each book after the first 4 of so is pretty much a continuous series) in the back. As the books also have anecdotes and essays after the true end of the book (which is disguised to surprise the reader), you can read an essay on the book you just read, then accidentally spoil yourself for the next book.
Daniel Handler (also known as Lemony Snicket) wrote The Basic Eight, which is really enjoyable and has a great twist. Unfortunately, at least one newer edition spoils this twist by stating that Flannery is not a murderer, but a murderess. For the record, Natasha did the murder but reading the back kind of gives avay that Natasha doesn't exist.
Jeffery Deaver's novel The Blue Nowhere: the book cover for at least one Italian edition reveals facts which happen halfway through the book, e.g. chief Anderson is easily murdered by the serial killer and Wyatt was an old friend of the serial killer.
Bridge to Terabithia spoils the death of Leslie on the back cover, in the summary on the copyright page, and indirectly by the Newbery Medal on the front cover. Newer editions of the book have a more vague back cover which only mention something tragic happening, but still have the copyright page spoiler.
Roald Dahl's Charlie and the Chocolate Factory: In an Italian edition from the late 1980s, the book cover reveals that Charlie will find a Golden Ticket. Well, that's pretty obvious. But it goes on further, revealing that the children, (cite) "one by one, will meet a dreadful fate, according to their flaws. The last one (who?) will become the new owner of the factory". The whole plot and ending spoiled!
The Chronicles of the Imaginarium Geographica: A printing of Here, There Be Dragons states on the back cover that the three main characters are, in fact, J.R.R. Tolkien, C.S. Lewis, and Charles Williams, which is not revealed until the very end of the book.
Circleverse: As per usual for Tamora Pierce's books, The Will of the Empress has a nice map at the front showing the geography of the fictional country where the action takes place. One location is clearly labeled as the place where Shan ambushes and kidnaps Sandry. This is both a twist and the catalyst for the climactic conflict of the book. Many fans were displeased.
Do not read the back of the Codex Alera, as there are some... rather surprising developments that can be spoiled if you're reading one book at home and decide to skim the back cover while waiting in line.
Another Henry James example: in at least one edition of Daisy Miller, the blurb reveals that Daisy dies at the end.
At least one Polish edition of Death Comes as the End has a back-cover blurb that spoils Esa's death, while a different edition has a excerpt from Henet's death scene. Both these deaths only occur towards the end of the novel.
Except for Guards! Guards! which gives away the fact that the dragon is crowned king, which doesn't happen until about halfway through the book and is apparently intended to be a surprise twist.
In Robert A. Heinlein's Double Star, you are given a cover that says that if the kidnapped politician John Joseph Bonforte doesn't reappear, the Reluctant Hero Lorenzo Smythe will have to spend the rest of his life impersonating the politician, which is how the book ends. This appears to be just wrong when you read the book, as the bad guys don't dare kill Bonforte, nor fail to return him after his double completes the ceremony of adoption, and Bonforte is in fact returned. In the end, Bonforte dies after the bad guys set up a grueling political campaign. The protagonist does in fact take up the impersonation permanently, as he realizes that only the man he is impersonating can keep his political coalition together, and Lorenzo had come to believe in the values of his "subject".
For some editions of The End of Eternity by Isaac Asimov, the back cover clearly spoils that Noys was sent back from the distant future to stop Harlan and the Eternity. Thanks a lot!
One of the most awful examples in history occurs in The Farm, where the blurb destroys the bait and switch carefully shaped by the author: The book sets up one of the two main characters with mysterious powers, then reveals in the final chapters that it's actually the other one who has them. Unfortunately, the blurb reveals who really has the power straight off the bat. Not only did the blurb spoil, it was also wrong, when describing the plot, aside from the massive reveal.
Isaac Asimov's Foundation Series: The European back cover blurbs for each book describes, in a fair amount of detail, events that only happen near or at the very end of that book, which leaves the reader very confused for a while ("This isn't about what the back cover said it would be about!") and then very annoyed as soon as it becomes obvious that the climax of the story has been spoiled.
Do not read the back of The Girl Who Played With Fire. It treats a major shock and story shift that occurs halfway through the book as the sole plot point, and casually mentions it as if it happens in the first couple chapters.
The Glass Spare: The blurb casually mentions that one of the characters is "a cursed prince". The existence of his curse is not revealed until over two-thirds of the way through the book.
One particular edition of Gone with the Wind summarised the events right up to the very last chapter, ending by saying: "When their daughter dies, Rhett leaves his Scarlett forever."
"Oh, look, The Great Gatsby! I've always wanted to check that out! Let me see what the book is about." *reads the back cover* "Sold!" *about halfway through reading the book* "So...why did the back cover spoil Gatsby and Daisy's relationship?"
The back cover for Guardian Cats and the Lost Books of Alexandria states that Marco's journey begins when "otherworldly creatures begin roaming the [library] stacks after night" and his mentor is killed. Cicero dies in the last 40 pages of the book and the stacks come alive after that. The synopsis ignores the first 3/4ths of the book.
This, however, was subverted by the cover text for Harry Potter and the Deathly Hallows. It says, though not in so many words, "Hey, this is the seventh book in the Harry Potter series. Either you're reading this while waiting in the checkout line to buy it, or you aren't interested in Harry Potter and thus aren't ever going to read this. So there's no point in having an advertisement here."
The ninth Haruhi Suzumiya novel, The Dissociation, introduces a new character, Sasaki. There's a big spoiler about her in the blurb of the English translation.
The movie tie-in version of I Am the Cheese has a back-cover blurb which tells you exactly what the situation with Adam is, stopping just short of the using the term witness protection program. This completely trashes the disjointed narrative and the feeling of confusion and paranoia it's meant to create.
The Kid Who Ran For President. Somewhere in the beginning, it mentions something about looking at the last page to find out the ending. When you do exactly that, it says something along the lines of 'Hey! Read this in order, you loser!'
J. R. R. Tolkien, it seems, hated the name that his editor gave the third volume of The Lord of the Rings, The Return of the King because it gave away one of the major plot points: the fact that Aragorn, finally, decides to accept becoming the King. Of course, it has a double meaning, it could easily hint to a Downer Ending of Sauron getting the ring and rising to power.
Nearly all Italian editions of The Lord of the Rings have an Introduction by Elémire Zolla (Italian literary critic, essayist and philosopher). Initially it looks just like it is a preface, comparing Tolkien's masterpiece to other famous works of the past... but at a certain point it starts talking about the plot, and before you can realize, in about 10 (TEN!) pages it has summarized the whole book, revealing the main plot twists (e.g. Gandalf's death and rebirth) and the twisted ending - you know, the one that's not in Peter Jackson's film (Saruman attacking the Shire).
Messenger has a back cover where everything is revealed, right down to Matty's heroic sacrifice, which only comes up on the last page of the book. And of course, when discussed in class, the teacher will mention not to look at the back panel. So of course, everyone does.
Mogworld. The first thing anyone learned about it was the world is an MMO. This fact is heavily hinted at, but doesn't get confirmed until a third of the way through the story.
A HarperCollins edition of Murder on the Orient Express spoils a vital twist on the back cover blurb, by stating that Poirot must find the killer "among a dozen of the dead man's enemies" thus immediately revealing that every single one of the apparently-unrelated passengers had a personal vendetta against the victim (something which, in the novel, is only gradually revealed towards the end).
There's a historical whodunit out there called The Mystery Of The Roman Ransom. The back cover tells us that it's about a group of boys in Ancient Rome who uncover that a respected senator - one of their fathers - is going to be assassinated, and that they have to find out who it is. Fair enough... and then you flip open the cover and see a lengthy excerpt from the scene where they find out who it is on the very first page.
If you're about to read George Orwell's Nineteen Eighty Four, don't look at the back of the book, which will probably quote the final line. The trailer for the film does this as well. Seriously. AVOID.
Fred Saberhagen's Octagon carefully conceals the fact that the real culprit behind the alarming events is a spontaneously-arisen AI that doesn't know killing players isn't the proper way to win a game until its fourth-from-the-end chapter ... except on its rear cover, which openly declares that the player who's out for blood doesn't have any.
The godly parentage of the titular character in the Percy Jackson and the Olympians series is saved for a big reveal several chapters into the book, but the back cover tells you right off that it's Poseidon. Not to mention the title of the series and even the vaguest summary give away the a-few-chapters-in reveal that the Greek gods are still around.
The advertisement in a Scholastic book order form blatantly says in one sentence that Percy is the son of the sea god.
The 1993 Baen Books printing of Podkayne of Mars by Robert A. Heinlein features a contest on the back cover where readers wrote in on whether the main character, Podkayne, should live or die. Apparently, Heinlein wrote the latter but his editor forced him to change it to a happier ending, completely undermining the entire point of the book. This edition featured both endings, as well as choice letters.
A cheap supermarket paperback thriller called Rabid, about the rabies virus getting into the animal population in Great Britain, one of the few completely rabies-free places in the world (and thus a place where pets are not rabies-vaccinated). In a twist at the very end literally on the last page of the book the virus mutates into an airborne strain. The back-cover copy ended with, "And when the virus mutated, became airborne, the whole world would learn what it was to become ... RABID!"
David Eddings' book, Regina's Song features not one, but TWO double-paragraph plot summaries on its back blurb. Both of them, in trying to be mysterious, blatantly state who the killer is and to some extent, what happens after we discover that fact. The book is pretty enjoyable, but still.
That would work a lot better if the stores didn't put the book's sequel with a spoiler as its title (although it is kind of obvious).
Rivers of London: The blurbs of the UK and US editions of book 2, Moon Over Soho, give away the twist, revealed fairly late in the book, that protagonist Peter's father Richard "Lord" Grant, a failed jazz musician, has a connection to the string of mysterious deaths of jazz musicians, a fact that gives Peter the impetus to solve the mystery.
Subverted in Shades of Grey by Jasper Fforde, where you can come across an announcement for the as yet unpublished second part that "De Mauve and Brunswick will return" for the second volume. Only you don't even find out until after the climax who they actually are.
All of the later Sookie Stackhouse books seem to have a compulsive need to spoil major plot points for earlier books on the back cover. Add in that all of the books look similar, so it's easy to pick the wrong one, and you've got a recipe for frustration. Extremely annoying if going from an early book to a far later one. Fairies? We don't even have werewolves yet! Vampire War? What? Oh- thanks for telling me that Eric wins it. Dammit book.
Through the webmaster of his official website he claimed to not have an Internet connection himself Terry Goodkind, author of the Sword of Truth novels, actually warned his fans that the cover blurb of book six was disgustingly spoilerish and not to read it before they read the book.
In several recent Penguin printings of A Tale of Two Cities, the plot twist that is not revealed until the last 15 pages is right there on the back cover.
It is almost impossible to find a cover for Tuck Everlasting that doesn't ruin the surprise.
The Turn of the Screw. Peter Quint's dead, and so is that governess he was dallying with. This is made out to be a surprise in the book. Thanks, blurb.
Many editions of Twilight mention in the blurb that Edward's a vampire, thereby robbing the first two hundred pages of any sense of mystery. Of course, if this hadn't been spoiled it would have been a pretty bad case of Genre Shift with a mystery romance novel suddenly including vampires. Imagine how that felt to the eight people in the world who didn't know about this beforehand.
The blurbs on the omnibus editions of Lois McMaster Bujold's Vorkosigan Saga are particularly bad, although the task is made harder by the blurb needing to be for at least two books at once.
The dust jacket of Warbreaker ruins a major plot twist if you think hard enough, by telling you flat out that Vasher is the titular Warbreaker, which is a major hint that Vasher is also the similarly named Peacegiver.
If you skim the back of any Warrior Cats from the end of the first series on before reading the rest, you know there's a cat named Firestar. After learning the naming conventions, it's pretty obvious who's going to become leader and in which book as well...
Add in that the first few book spoiled name changes and deaths as well, in the first few pages! Erin learned her mistake and either tdid not after a certain point in the book, or baited us in.
The back cover of The Westing Game boasts that only two people know all the clues to solve the Game: a Westing heir, and you (the reader). Way to go, revealing that one of the heirs is in on Westing's scheme being Sam Westing himself...
However, the twist is still there as to who it is and how they know.
Some versions of The Wheel of Time books are odd about this, as they give away plot points that only become relevant for the next book.