new back pack?

I need to buy a backpack. The $20 backpack I purchased at target isn't working! Yesterday, I carried 3 textbooks to school so that I can study at the library. Last night while I laid flat on my back, my back and shoulders were hurting very badly. I was doing excessive lifting and swinging of the 20# bag, not zipping much though. To Gavin's electronic geek-ness, his suggestion was to find an electronic version of the textbook, which I wasn't able to find in the publisher's website. So, here is a pack I found at REI that might work. Let me know what you guys think.