Ah, purchased from Audible! That may well be the problem now after your recent iOS update. I'm not a user of Audiobooks but from what I've seen, Apple has been rationalising its relationships with a number of developers/organisations recently, of course to concentrate on its growing new music services.Okay I did that, I found the new Apple Music app, and all my music is there.
But cannot find my Audiobooks, i saw a message that said books are in IBooks, when i go to IBooks it says no Books (my Audio books were purchased from Audible and not ITunes - perhaps this is the problem?)
Why must Apple make it so difficult to use their devices - It was all working perfectly before?