Okay so lets be honest, I read this book because I love the movie.
I must admit that I do not like the book as much as the movie. However there are a number of very interesting things in the book.
First, the bad, I found the sections that were not related to the story, you know where the grandfather is reading in the movie, very slow. The screenplay also changed the narrator in the movie. However, the thing I am most disappointed with, is that Buttercup is totally unbearable on paper...even more sappy than in the movie. I literally wanted to throw the book against the wall when she spoke, which is not often. For a book about her, there is very little with her in it.
Okay onto the fun stuff...the narrator in the book is William Goldman, the author, screenplay writer. It is a running dialog of his thoughts related to the book he is compressing. You get a little bit about his family (not sure if this is true) and a little bit about his job (I looked this up and it was confirmed, so when he talks about Butch Cassidy and The Sundance Kid, well, it is because he wrote it!)
I liked the back story on Fezzik and Inigo, they are after all in my opinion, what makes the story so good.
Here is what I am not sure about...my particular copy of this book has an additional chapter (maybe) called Buttercup's Baby. I really liked the story, could not stand his constant breaking in because it made the last 40 pages much longer than they needed to be...but it gives you a bit of insight into what does happen after "happily ever after"
All in all it was a fun read, I would not call it quick but if you love the movie you will definitely get some more depth from reading the book.