I've read the book, AND seen the musical.
LOVE THE MUSICAL! I even got Idina Menzel's AUTOGRAPHHH!!! YES!!!
The book is REALLY REALLY naughty. Like hard core. Like have you gotton to the part where Fieryo and Elphaba...um...nvm. Anyways its pretty descriptive.
The play has a TOTALLY different ending, and is REALLY not like the book. I mean there are parts that you will recognize. But for the most part there is major differences. Like in the play theres a relationship created by two characters that NEVER happened in the book.
I mean the Book is good adult read. But if you just watched the movie. The play is better for those who never read the books and just watched the movie.