As usual, let me start off by discussing the book. Like the movie, the book presents itself with two narratives (though the book's first narrative is a bit different). In the book, William Goldman sets up a gimmick. As a boy, William Goldman loved sports, had ADHD, and had no use for books. But then he came down with Pneumonia, and his father decided to read him a book called The Princess Bride, an epic Florinese novel by a man named S. Morgenstern. The book changed his life. He becomes an avid reader and, later, an author himself. He has his own wife and son now (who he's constantly degrading, but we'll get to that later), and he wants his son to experience The Princess Bride like he did. But his son couldn't even get through chapter two. And when Mr. Goldman sits down to read the book, he discovers why: this isn't much at all like he remembered it as a boy. There are endless pages of Florinese history and pointless passages of description that the scholars see as classic satire, but which he sees as dull. So Goldman sets out to write an abridged version of the book, the "good parts" version, cutting out all that pointless or dull stuff. And this is the book's gimmick. Every now and then within the book, there are short interruptions (in italics) where Goldman explains something has been cut and why, or he'll give a story of why his father had skipped these passages in reading to him in the first place and the conversations they had at these points in time.
But we also have the "main" story, which is that of true love, adventure, etc. I'm not really going to bother explaining it because, well, if you don't know the story of The Princess Bride, shame on you.
So how is the book? It's pretty good. The first 30 or so pages, though, is pure ADHD setup of the gimmick, bouncing back and forth between stories of Goldman's life--childhood and adult--and how he both learned to love reading and how he got to this current point in his life. But then it picks up and the real story begins.
The writing style is something you'll either love or hate. There are a lot of fun "statistics" (much like the movie's "there have been 5 great kisses...", which actually is in the book, though around the middle instead of the end). He also uses an incredibly amount of polysyndeton (where he connects many things with 'and' and it seems like a run-on sentence, when it fact it's just a really long one. It's a stylistic thing that most authors don't use anymore. You mainly see it in older works anymore. Hemingway did it frequently, as well). And there are a lot of parentheticals where it'll say something like (this was before THIS but long after THAT), some of which weren't historically accurate, some of which were, but that was the point (at least according to the abridged notes). And then there are the abridged notes themselves, which I didn't find nearly as annoying or tiring as I've read in other reviews. In fact, the majority of them end up in the movie in some form or fashion (with the sick grandson/grandfather bits).
There was really only one thing that surprised me about the book: it was mean. From either characters or Goldman himself, there was a surprising amount of misogyny, superficiality (primarily with women), racism, anti-Semitism, cynicism, and just all around insult-tossing. Honestly, the mix of the "true love" stuff, "perfect beauty" talk, and mistreating of women, there were numerous times I kept flashing back to Twilight, which was really rather painful to even think about making that comparison. Though, thankfully, the difference is that William Goldman can actually write, and write very well. And his characters are actually likeable--when they aren't being uncharacteristically angry for, sometimes, no reason. And that was another strange thing. There were few times in the movie where the characters seemed angry or upset. In the book, they got angry quite a bit, yelling, insulting, etc. Though some of the "angry" lines went to the screen and were acted with wit instead of anger, which is much better, in my opinion.
And to top it all off, the book has a horrible ending... if you can really call it an ending. The "Princess Bride" story just kind of ends (and the "unabridged" ending that's shown is also terrible and such a downer), but it ends with more abridged notes after that... which ends with such a cynicism that it nearly destroys the feel good "true love conquers all" mood of the rest of it.
But then we have the movie. If the book is the abridged "good parts" version of the S. Morgenstern work, then the movie is the "good parts" version of the abridged work. And ironically, the movie was also written by William Goldman. But besides even the writing, the casting is perfect. Cary Elwes, Robin Wright, Mandy Patinkin, Andre the Giant, Wallace Shawn, Christopher Guest, Chris Sarandon, Billy Crystal (et al)... perfect.
Now, don't get me wrong, in retrospect, there are some things better about the book, and some things better about the movie. But for the most part, this is an incredibly close adaptation. Some lines were altered from the book for the movie just slightly (mostly when things had to be condensed), and for the better. But there were also entire lines and scenes that were a bit different from the book that I actually prefer in the film (most specifically, how Westley presents the story of the Dread Pirate Roberts to Buttercup both before and during the Fire Swamp... in the book, it's not even brought up until after Buttercup realizes the man in black is Westley and they're in the Fire Swamp).
So what did the book do better? Character backgrounds. The movie, of course, briefly touches on Inigo's history in terms of his father and his revenge. But there's so much more in the book. Not to mention Fezzik's story is almost all but removed entirely (there are certain points where you can get bits and pieces, such as Fezzik's nearly indiscernible speech to Westley about fighting one person versus groups, or Vizzini threatening to send him back to Greenland). And because Fezzik's character is played down a bit, there's a line at the end that doesn't make too much sense and seems out of place, where Inigo says "You've done something right." Also, I was surprised how how much more rhyming there was in the book than the movie. But I'm rambling now.
Buttercup, Vizzini, Humperdinck, and Count Rugen also have more details to them than the movie grants. Actually, the movie takes what has to be the first 100 or so pages of the book and condenses it down into about 7 minutes. And what else to these characters have in common? They're actually less likable (including Buttercup) in the book. For Buttercup, that's a bad thing, but for the latter three, it's good... for the book, anyway. Primarily Humperdinck and Rugen, they seem much more villainous and dastardly in the book. Rugen is more calmly psychotic (he's really a great and disturbing character, and Christopher Guest played him brilliantly for what he was given). And Humperdinck's hunting skills are hugely downplayed in the movie.
In fact, that brings us to the biggest cut/change from book to film. The book's Zoo of Death becomes the film's Pit of Despair (not to mention The Albino's personality shifts completely for the little screen time he has, though I think for the better). The Zoo of Death is a 5-level containment area, where each level gets more and more dangerous. Humperdinck keeps the world's most dangerous animals there, and depending on what he feels like hunting at the time, he'll have The Albino go and get it for him, and he'll hunt it then and there. The only empty level is the fifth level, saved for the most dangerous animal, which he hasn't discovered yet. Turns out, this is where Westley is held later, what the movie turns into the Pit of Despair. Though the only real reason to have kept it in the movie would be the scene where Inigo and Fezzik have to travel through it to get to Westley, though those descriptions in the book were rather confusing and not nearly as exciting as it could have been (there were hippos and lions, and they end up fighting a giant snake and some bats).
All in all, the book is a good companion piece to the film, filling in some blanks to help things make more sense (though they made sense to me until I read the book, then I realized that there were just some things that were only touching the surface of what was really there). Of course, there are more things than I've mentioned, but nothing drastically important.
I suppose when looking at the two, I must say that the movie is better than the book. The movie does have some great things in it, primarily the more extensive character details, but the best parts of the book come in the humor and dialogue, most of which comes off word-for-word in the movie (seriously, as I said, this was a close adaptation). I did like the statistics in the book, though. If you like/love the movie, I'd suggest reading the book at least once. It really is a really good book. I just happen to prefer the movie.