Windows Transfer: MP3 Players
as of 7/17/15, only iPods are fully compatible with both formats of audiobook. Transfer to any other brand of MP3 player (Sansa, Walkman) from a Windows computer will only work for one format of audiobook. More information here.
Mac Transfer: iPod Only
Macs can only transfer to iPods. If you have a Mac and a non-iPod MP3 player, you will need to use the Manual Download option.