I read this book for English class this year and I loved it. It's everything I usually look for in a book: funny, good plot, and more layers to it than at first glance. People in my English class thought that S. Morgenstern was a real author; he isn't, William Goldman just made up a very elaborate backstory to the novel. There is no original novel which William Goldman condensed, and he doesn't have a son in real life: he has two daughters. A lot of people were caught, and they became almost outraged that William Goldman bent the truth in the introduction, because the introduction to a book is supposed to be grounded in fact and real life, not another piece of fiction. William Goldman was trying to suggest that people should be more critical about things they read and see, and that there could be more than meets the eye to any given thing.
The traditional fairy tale genre and quest narrative are deconstructed in the Princess Bride and made more flawed and geared towards adults. The characters are layered, and there's more to them than meets the eye, well, except for Buttercup, but that's another story. The characters are all people who have flaws, and none of them make perfect decisions. Buttercup decides she can survive without true love because she wants to stay alive more. There's not really a perfect fairy tale ending like there is in the movie, which makes it more realistic.
*runs off to find English notes*
The story has a central message of "Life isn't fair." It isn't fair by necessity because life isn't a fairy tale and there's no such thing as a happy ending, and it isn't fair because it won't be perfect, but it's still worth living. The ending of the story is happily enough ever after, and so it's a "realistic fairy tale". William Goldman is challenging readers to change their perspective about the plot and characters in a fairy tale, and our understanding of the real world.