Depending on how heavy those books are (and I know how heavy even one can be!), plus the other stuff you will most likely bring, you will more than likely want to distribute the weight into two panniers.

Or get a large basket and mount it to the rear rack. Drop backpack in, cover with bungee net, and be off. Or do what I did. It's not a basket, but it was free and increases the surface area that my backpack can use without smushing it too badly (and less chance of straps getting caught in the spokes).