Do you need the actual physical books for OZ? or are you just wanting to read them? or do you want to read them to see if you want to own a physical copy?
Project Gutenberg has quite a few titles for L. Frank Baum. Here's a link:
If you don't mind reading on line, or if you have a device that supports these files... These are public domain books. Lots of very old, interesting items... and some rather new interesting books.