Re the backpack can you explain more why you need so many books? Is there no locker system you can take advantage of?
You are already doing about the best you can to carry a heavy load. A barrel bag that you hold in one hand on one side would of course be disastrous. Hanging a bag over one shoulder is also bad. Spreading the load over both, right up on the torso, is the most effective and safe way to carry it.
If you can't avoid carrying it at all, make sure you don't carry it far. The minimum distance possible, and if that's more than a few minutes walk between points, stop on the way, put it down, and rest for 1 minute.