The difference between it and the book are almost the same as if they had made a movie instead of a musical. If there is a movie of a book I always recommend watch the movie first. If I've already read the book my mind tends to pick out all the little things that have been missed or change.
Whether or not you will enjoy the book depends on how much you know about Oz - forget the Judy Garland version and the subsequent attempts to reinvent it - I mean the real Oz - the books by Frank L Braum. Wicked - The Life and Times of the Wicked Witch of the West has taken that world and realized it for the adult mind with hostile takeover, corrupt government, resistance forces and magic as well as mixing in the more tragic aspects of a life trying to do what is right.