Watch out on BooksonBoard audio books. Some of wma which are drm'd. The mp3's are drm free though.
Checking out at online libraries you can get either mp3 or wma, again mp3's are drm free and wma's have drm.
The only site I purchase my mp3 audio books from is AudioBookStand.com. They deliver an mp3 disk, or several cd disks if you prefer that format. AudioBookStand.com also has a sister site that you can download books, but they are not mp3's but they do work in readers just fine, I believe. I've never downloaded from them.
Librivox is gutenberg.com's audio books that volunteer's read. Not professional reader's but all are acceptable.
Audible.com is another site, but I don't like their terms so I don't buy from them.