It looks like the deal is dead prematurely. Here are the coupon eligibility requirements and it is supposed to be good till tomorrow:
Online Coupon C3X7J9K Details:
This coupon can only be redeemed once per customer on a first-time purchase with a new account online at bn.com.
This coupon is valid from October 6, 2009 to November 12, 2009 at 2:59 a.m. Eastern Time.
The 20% discount will be applied to the most expensive eligible item in the order - not the entire purchase - and will be deducted after all other eligible discounts, including (if applicable) the standard Member discount, are applied.
Additional Rules Governing Use of this Coupon: This coupon is (i) not redeemable for cash or cash equivalents (including Gift Cards and online Gift Certificates); (ii) not valid on past purchases; (iii) not eligible for purchases of the following products and services: Barnes & Noble Memberships, Gift Cards, Gift Certificates, gift-wrapping, textbooks, video games, digital content (including but not limited to digital books, magazines, and periodicals), certain digital devices, products available for pre-order, products from the Rosetta Stone Series, products available for in-store ship-to-home orders, shipping or handling expenses, or products marked as not eligible for "coupons or promotional discounts," (iv) not valid on purchases made from third parties accessible from the bn.com website (e.g., used books, PC and video games, etc.); (v) may not be combined with any other coupon; and (vi) may not be combined with the Educator program discount. This coupon is not valid at Barnes & Noble retail stores or Barnes & Noble College Bookstores. Void where prohibited by law.