An Infinite Number of Monkeys

But if you haven't, you should absolutely read Wicked, by Gregory Maguire. It's absolutely brilliant (though I didn't like his second book, on Cinderella, nearly as much). For another, very harsh take, read Was, by Geoff Ryman. It's almost hard to get through in places, but is very, very good, and is a fascinating take on Dorothy, the idea of Oz, and even Judy Garland.