I searched these forums and several people have the same problem. No solutions that I have seen.
Apple had very little to say on it http://docs.info.apple.com/article.html?artnum=61695:
"Do not connect an iPod with Audible files to a computer that has iTunes 2. If you do, iPod will lose the ability to play Audible files. To fix this, connect iPod to the computer you first used to download Audible files to iPod. Once iPod appears in the iTunes Source list, it will be able to play Audible files, but you will need to copy the Audible files to iPod again."
Apple also stated http://docs.info.apple.com/article.html?artnum=61656:
"Important: In order to play Audible files on iPod, you must install iPod 1.2 software or later."
If your software is up to date you should be running iTunes version 6 and if you go to Settings->About on your mini it should say it is at 1.4.1.
Have you considered creating a playlist for your Audiobook? This seemed to be the most popular work-around for the issue. In iTunes select all of the files in your Bible, and click File->New Playlist from Selection. Synchronize your mini so that the play list is on your iPod.
When you want to listen, use Playlists->NIV Bible-><file you want to hear> If Playlists are not enabled at the top level, you can turn it on in the settings Settings->Main Menu->Playlists. Or you can use Music->Playlists->NIV Bible.