Sounds like, in the Osprey line, the Aether or Argon would be what you need. (I just realized that I leaped to the conclusion that you are male. If you are female, their Xenon and Ariel series packs are the equivalent.)

Being many years removed from school myself, I can't imagine needing anything that big for books. I'd suggest you get something specifically designed for that, in the 25 - 40 liter range. (Frankly, with technology being what it is, and e-reader software so proliferous, I was a bit surprised you're still carrying books.)

Actually, I just noticed on Osprey's web site that the Argon and Aether series packs are "Add-on Compatible," which as I understand it means you might be able to buy a pack on the low end of your capacity range, buy a separate day pack/book bag, and lash the separate pack to the large pack for the extra capacity you specify. That way, if you do take trips where you don't need the extra capacity, you can leave the smaller pack behind and not have to worry about how to compress the larger pack. Just one more thing to think about.

As far as waterproofing, I doubt you'll find one that is truly waterproof. Better options are a waterproof liner bag or a pack cover. (A liner is just a pack cover that goes inside the pack, into which you load all your gear. Some people use heavy duty garbage bags, and carry a spare or two in case of tears.) Osprey sells pack liners separately in different sizes to fit their various packs.