If you are receiving and alert that media will be erased when syncing the device, this usually means that media on your iPhone was synced with another computer. If this is not the case, the computer in question had some hardware changes and iTunes recognizes it as a different computer.
If the Audio Books on the device were purchases via the iTunes Store, you can transfer the media from your iPhone to the computer via the steps below.
Only iTunes purchased content will transfer from the iPhone to the computer.
Hope that helps,
I updated iTunes using the automatic update reminder that I get regularly. Often the update does not go smoothly (as it did not this time) and I get an error message. I don't recall the number but it says something like "Error Code: 2324". When I get that code I found that I need to uninstall everythng Apple: iTunes, QuickTime, Apple Software Update, Apple Mobile Device Support, Bonjour and Apple Application Support, I then do a clean install from the Apple website. Before now when I did so I automatically got back everything: my podcastes, my Audible audio book library, my iTunes media downloads, etc.
When I went through the steps this time I only got my iTunes purchases back. I am using the same computer that I always use to sync my iPad/iPhone with iTunes. I have added no new hardware/software since this process began with the exception of the clean install of iTunes.
If I try to do a sync between my clean install of iTunes and my iPhone I get the message, "Are you sure you want to remove exinsting movies, TV shows, books and tones from this iPhone and sink with the iTunes library?" Since the iTunes library is blank, I can't do that. I had hoped that iTunes kept my files in a safe place that would not be affected by my failed update. That is a common process with many/most programs.