I read the book before I saw the musical. I agree with you--the book is REALLY weird, and pretty dark (I don't know if you've gotten to the part where they visit a seedy sex club, but that scene most definitely didn't make it into the musical
The book is also extremely different from the show. I read once that the musical was an adaptation of the book for "fourteen year old girls," and that makes sense. I prefer the show--it's so much happier!