You can save 40% on one book online when using a Mastercard. Simply enter code X8X9X3K at checkout when shopping at Barnes & Noble.

From the website:

This coupon can be redeemed once online at BN.com per customer. This coupon is not valid at Barnes & Noble College Bookstores or Barnes & Noble retail stores. Full payment must be made with a valid MasterCard card. The 40% discount will be applied to the online price of the most expensive eligible book in the order — not the entire purchase. For Members, the discount will be in addition to Member discount.

This coupon expires at 2:59 a.m. Eastern Time on January 1, 2008.