I have not enjoyed a book so much since Eats, Shoots and Leaves, which I read in February 2010. This one just had me smirking, smiling and reading passages aloud to Eric nearly the whole way through.
You've probably seen the movie, so I don't feel the need to elaborate too much on the plot. By its own description, it is a "tale of love and high adventure." It's also incredibly clever and witty.
Having seen the movie many times I anticipated that I would consider the book to be superior. It almost always happens that way; you rarely hear the phrase, "Oh, the movie was so much better." In this case, I don't know that I prefer one over the other. The adaptation to film was perfect. The scenes that were altered from the book's original story were done in a manner that was better suited to film. Yet they still stayed quite close to the original book.
There were numerous portions of the book that were basically put straight into the movie, particularly with the dialogue, and I loved those portions because I could really hear the voices and accents and intonations in my head.
I also love that the book provides great background on all the protagonists - Buttercup, Westley, Inigo and Fezzik. I particularly loved learning more about Inigo and Fezzik, who are easily my favorite characters.
My favorite part, though, is the fact that the book (though the image I found for the cover doesn't show it) is generally subtitled: Based on the Classic by S. Morgenstern. Goldman acts as though he's adapting a work written by another, when in fact the whole book is entirely his. (Morgenstern is a pseudonym.) Because of this the book is full of his own italicized comments and personal stories, and they are as funny as can be.