I tested audiobooks on my old iPod Touch.
Could not get library Overdrive app to run because my iPT is too old to update it. However, that does not matter to you because you have MP3 files.
So I tried to drag some MP3 files from my computer to the music files on the iPT. No dice, but maybe just because I am out of practice.
Then, I got iTunes to take some MP3 files to the iPT by asking it to add them to Documents. Bingo. The files went there. They play on the iPT.
Also discovered along the way that you can download audiobooks from iTunes, so perhaps you could put your existing MP3 audiobooks there and then onto an iPod or iPT.
I hope this helps you.
2006 Tiger CX 4x4, 8.1 L gas V-8, Allison 6-speed