I agree with others above who have recommended the cybook
I bought mine earlier this summer--and wanted it for the same things that you do (although I've never tried an audio book on it)--and ended up choosing the cybook over the sony because of book availability. Selection seemed a lot better for the cybook.
Also--just to clarify--the cybook does come with a case. It's a relatively soft case, but it protects against minor bumps and scrapes, and makes holding the cybook feel more like holding a paper book.
Connie