You could try putting some audiobooks on a microSDHC card in the player and see if they will play from there. If so, then you might be able to take the card out and put a different card in the player and load other books on the second card, then swap the first card back in and see if the files will play from the first card. Putting protected files on card memory sometimes will work, but sometimes the files are unplayable from there. Turn off the player before adding or removing a card.
Does Overdrive let you select what bitrate the files will be on your player at? I use 32kbps mono mp3 for my spoken word files which sounds just fine. That way the files take up just 1/4 of the space they would use at 128kbps. my spoken word files though are usually podcasts(which unfortunately are sometimes posted at 64kbps or 128kbps). Only a few kind websites post their podcasts at 32kbps.