LibriVox.org is a big repository of audiobooks based on works that are in the public domain. Thier services are free. You can download anything you like in the extensive catalog for free. And while one can download an audio book and then convert it, would it not be easier to somehow integrate Nokia audiobooks and LibriVox?
One of the reasons why the iPod became so popular were podcasts. Content and it's delivery. Not just a player. This way you can provide an extensive book cataloge and maybe even at a later date add a store for audiobooks that are not free.
All the books in the cataloge are also available as rss feeds/podcasts so that might be one way of going about it...