Having found many of the works of Frank L. Baum available as free audio books via the awesome Librivox, we have been downloading them for the kiddos to listen to in bed at night. Check out the wikipedia article for a full list of Oz books (including cover art!) Librivox also provides links to relevent wikipedia entries, archive.org content, and even a “Chapter a day” service you can subscribe to.

Librivox Search for “Frank L. Baum”