First, I would suggest getting a pack for school, and a different one for backpacking. Your price range is high enough that you can easily do it. For carrying a lot of books, you want a pack that is made from heavy materials. Most of the Osprey packs won't take the abuse of heavy books being set down on concrete (which is what you will do with your school pack). In college, I had to take a laptop, and all my engineering books around. I used a really heavy duty, large day pack that I bought while in the Army. I ended up destroying it. It was made of 1000D cordura and built tough. It even had an internal frame.

For your large pack, and based on what you think you want, I would suggest an external frame. Look at the Kelty Yukon, or something similar. It will be more flexible, have more pockets, and weigh less/cost less than another
really large, heavy internal pack.