The musical "Wicked" is based on a book that was in turn based on what other book?

Wizard of Oz my all time favorite. used to watch it every year with my late sister she loved it as well.

I've read the book but never seen the musical. Every time it comes around, I'm broke.

I never heard of wicked before the south park episode. I thought it was made up by them.

I see the play in New York in 2017, very Good.