I read the book and saw the musical, and I looooooved both. They are slightly different, so if you've only seen/read one or the other, then be prepared for that when you read/see the other.
For those who aren't familiar; Wicked was originally a book written by Gregory McGuire that tells the story of the Wizard of Oz from the Wicked Witch of the West's perspective. It has since been adapted into a Tony award winning musical. I highly recommend both. Also, Gregory McGuire has several books written from similar angles on other fairy tales.
This musical is also discussed on Greekchat
here.