When I have to purchase books, I shop around. There is no need for me to pay full price for books. Some of the sites I frequent are:
Barnes & Noble
These sites have their sellers display cost and shipping right up front to reduce suprises at the checkout. Please just take 2-3 minutes to shop around before you purchase. A dollar saved is a dollar earned.
This also works for movies - you can save 50% over Wal-Mart.