This article was the result of a really frustrating ten minutes of trying to get a book on to a child's ipod. Audible customer service wrote back to me, but by then I'd figured it out. They were nice though.
Audible Audiobooks on iPod Shuffle, iPod Nano, and iPod Touch
Audible.com offers different formats of audiobooks to download. Choose a format based on file size, iPod model, and iPod generation.