Playing audio books on iOS 5
The Audible app will play any audiobooks in your iTunes (now Music) library on your iPhone. To make this work, select the "My Library" menu item and then click the button for the "iPod Library". Clicking either "iPod Library" or "Audiobooks" will then show all of the books in your library, including non-Audible.com books...i.e. books synced from iTunes.