William Goldman's 1973 "good parts" version was, of course, written by him and not by S. Morgenstern. It was a best seller and remains a cult classic today. And it also was made into my third favourite movie. If you haven't seen it yet then there's something wrong with you.
Yes, the book is the classic tale of true love, high adventure, pirates, princesses, giants, miracles, fencing and R.O.U.S.'s that we are all familiar with. But the novel also has Goldman's fictional commentary about his abridgement of Morgenstern's original satire on the excesses of Florin royalty and his explanation of what he cut out and why, including a whole chapter just on hats. Although not as funny as the film, it does have the important back stories missing from the film. You could read it just to see how Buttercup came to be viewed as the most beautiful girl in the world and how it made her vain until she realized what "As you wish" meant. There's also the tale of why Westley loves Buttercup and what Fezzik's life was like before he met Vizzini. There's also the several other princesses that Humperdinck considers (this is where the importance of hats comes in) before he meets Buttercup. And the original location of the Machine is revealed (hint: it's in the Zoo of Death). There's also the heart-breaking tale of the death of Inigo Montoya's father.
Read it and you'll watch the movie in a whole different light.