Have you read the book 'Wicked' yet? It's really good and is the start of the 'The Wicked Years' series. I haven't gotten my hands on the rest of the books yet, but I loved Wicked for it's version of the origin tale of the Wicked Witch of the West and how it opens up the world of Oz to us.

The other three books in 'The Wicked Years' series:
Son of a Witch
A Lion Among Men
Out of Oz

And yeah, I know what you mean about wanting to write Oz stuff.