While Ortleib panniers are very tough, the backpack harness is next to useless and takes up a lot of room inside the pannier when not being used.

I've found that the Arkel Bug is a much better solution, and is just as tough as the Ortleib with a much better design as a backpack and many more usable pockets.

I own all and have tested all of the products mentioned above.

EDIT: The Bug is big enough to carry all of your books; you really don't need a double pannier.