Hi Mimi,
I just saw your message now. As far as iTunes goes - it looks like your client will need to complete an application to become an iTunes Content Provider. More information can be found here:
http://www.apple.com/itunes/content-providers/
Also, you may want to try checking into Audible.com as a place to list your client's audio book. Audible.com is one of the more popular places for downloading audio book content on the web. There are other sites too, like iAmplify.com
Good Luck!
Darlene