I was talking with my sister and her boyfriend about the upcoming college semester today at Sunday dinner. As they were lamenting the high prices of college textbooks, a light bulb went off in my head, and I vaguely remember reading something about an online service that allowed you to "rent" textbooks for the semester. It was cheaper than buying textbooks used, and they provided a postage-paid return envelope so you could send the book back at the end of the semester.
I did some searching through Google Reader, but couldn't find the exact article that pointed to the service, but I did find two companies that seem to fit the description: Chegg and BookRenter.
If you're looking for something similar for any kind of book, try BookSwim instead. If you want to buy instead of rent, try Zooba.