I've just bought an ipod shuffle and loaded an audiobook on to it which was fine. I then loaded some music and whilst it appears in the itunes book folder I can't work out how to listen to it on the ipod shuffle?
Take a look at this Apple article about using VoiceOver on the shuffle
in particular, the parts labeld Changing playlists and To choose an item from the playlist menu.
So, I think you press and hold the VoiceOver button (on the shuffle) to hear the list of playlists. Select the playlist you want, which in your case is an audiobook.