http://www.amazon.com/Dowco-Rally-Pa.../dp/B001A6JH02
I've used this one a couple times. I modified it so I just clip it under the passenger seat, rather than looping the other ends around parts of the bike. It doesn't convert to a backpack though, and I'm not sure if you'll be able to fit 4 books. I've put 2 books stacked (about 1.5-2" each) and had some extra room though.
Here's the pictures of it.