On which platform do you listen to and download audiobooks? If you are an Audible user, you can only play them on limited devices because Audible files have DRM protection. If you want to play your audiobooks on many players without limitation, you should use some tools to remove DRM protection first.
Speaking of the useful tool, you can use DumpMedia Audible Converter to remove DRM protection and convert your files to some common formats. It has a fast conversion speed and won't cause quality loss.
After converting, you can play your Audible books on many players. And you have many choices to select.