Using Overdrive (library system) and audiobooks

Another thing to try - on your player go into Settings|System Info|USB Mode and switch it to MTP.  You may need to retransfer the books in MTP mode.  I use my Clip/Clip+/Fuze players almost exclusively with Overdrive (and mine work in MSC mode, too, but MTP is supposedly “required” so it’s a good thing to try).