Of course there is the obvious way to get lots of MP3 audiobooks....make your own!!!
I purchased the complete unabridged "Lord of the Rings" audiobook on (regular) CDs. It added up to a total of over 40 CDs!!! I just ripped them, with EAC, and encoded them, with LAME, at 64kbs. This is probably overkill for voice, but what the hey, they sound great. This lets me condense the complete trilogy down to a few mp3 discs. I have also recorded books on tape to wav files through my soundcard and then encoded them into mp3. It is a bit of effort but once it is done, it is much more convenient to have the compressed versions.
Try it, you'll like it!