I think you should begin putting audio books on slot radio cards. Sansa mp3’s are the best for compatability with audible and this would be another enhancement to the versatility and paortability of audio books. Would be similar to the way you put musical albums on slot radio cards and make the rest of the card available for recording by the owner.
Personally I don’t listen to audio books, but I think this is an excellent idea if the licensing and/or copyright aspect of it can be worked out.