It's been a while, but I sold back any still useable textbooks via Abebooks.com. Then there are the book recycling bins some schools use as a fundraiser. And finally, I've dropped more than a few boxes of magazines off at the local recycling center.
Math is tough. Let's go shopping!
Goodwill- tax write off....
http://www.barbedwirebooks.net/
I go here with used books. they only give store credit, but its a good place.