I put my audiobooks on an 8GB Sansa Fuze, even though my Libre Pro can handle them. They are just more manageble that way, I can put more on the Fuze, I can easily pick up where I left off on each ebook, it's lighter and easier to carry when walking, and when I play the audiobooks through my car's stereo I can put the player in one of the little niches designed for a garage opener remote which keeps it out of the way.
Remember, too, that audiobooks sound fine at 64kbps, so there's no need to keep them at the size you would for a music file. That allows you to load more audiobooks on your device.