Published in 1973 by William Goldman, I found The Princess Bride spiritedly, engaging, and altogether charmingly humorous. I found the reading easy, the humor congenial, and the characters all a bit wacky, which is par for the course for the late 60's/early 70's.
William Goldman himself was a screenwriter by trade, creating the book from his screenplay. I think it's fair to say that this is one of the best written fantasy works that I've encountered from the 70's, not quite resembling anything before or since, except Douglas Adams' The Hitchhiker's Guide to the Galaxy. This comparison is well noted, for both he and Adams were screen writers who knew enough that you can't do a straight adaptation of a humor film to a book. To create a book, you need humor that works as a book, not just humor that works as in a film. So the book itself is filled with many jokes that never appear in the film.
Goldman asserts that the The Princess Bride is a translation of a book written by an Italian writer back in 1931. The unabridged work is long, farcical, and utterly tedious, so he abridges it into the more readable book that we see. Along the way, he explains why he took out certain sections and complains about the original writing, often telling tangential stories along the way. With this conceit, he is able to fill out his manuscript to proper book length without having to alter his story at all.
In my version, he also includes an introduction which talks about the film, and a purported sequel, Buttercup's Baby, but only first chapter.
In all, I was quite pleased with the read. He captures the mood and the fun of the film years before any actor had been cast. Or perhaps, I should say that the film nicely captured the humor and mood of the book, which was based on the screenplay.