On my library's Overdrive site, you can now buy books! I imagine all libraries are offering this if they have Overdrive, but anyone can buy from it. The prices aren't bad either.
http://www.librarybin.com/2D0790DB-C...en/Default.htm
It looks like they offer all the Overdrive books, audio and ebooks, not just the ones my library has. Too bad they don't 'rent' them too!