Wicked: The life and times of the Wicked Witch of the West, by Gregory Maguire
The most excellent revisionist Oz book so far. Maguire revisits the Oz mythos of both the books and the '39 film and reinvents the evil character into a fascinating creature. Part study of the nature of evil, part study of religious fervor, part study of gender roles, and all fascinating.
If the Baum books annoy you, but you have a little hankering for the movie, read this. If the movie bugged the hell out you, but you like the whole idea of Oz, read this. And if you like fantasy with a good helping of cynicism, read this.
This was the first novel by Maguire (Geoff Ryman's first novel, Was, was also themed on Oz). Macguire's second book is another fairy tale revision: Confessions of an Ugly Stepsister.