Wicked, a book by Gregory Maguire that spawned the Broadway musical (which I still have not seen).
This book is a sequel to Wicked, picking up where it left off -- in particular (SPOILER ALERT) after the Witch is killed by Dorothy.
Liir is the main character of this book. There was some indication in both books that he is the Witch's son, but never was there definitive proof in either book. I kept wishing they had a way of doing DNA tests in that fictional world, but alas.
Anyway, the book talks about Liir's journey into adulthood and how he created a life for himself out of a past he didn't really have, since it was so shrouded in mystery. He interacts with many interesting characters (the Oz people like Dorothy, Scarecrow, etc. included).
Another one of the main characters in this books is Candle, a mysterious girl who helps heal Liir from the edge of death, and she is ultimately very important leading up to the conclusion of this book, which admittedly wasn't that happy of an ending. :(
At the library the other day, I saw another related book my Maguire called A Lion Among Men, which focuses on Brrr, the Lion in the Wizard of Oz story. I checked it out and can't wait to read it. I can't believe that I actually got into books in the Fantasy genre...