How do I use my gift card to buy an eBook?

This isn't very intuitive. Login to the B&N website on your computer and click "Account" at the top. Then on the right click "Manage Gift Cards for eBooks". Put in your gift card information. Now you can shop for books. You will be confused by "will be purchased with your default credit card" near the "Buy" button - pay no attention. When you click the button it will inform you any eCard balance will be used first.

If the purchase exceeds your gift card balance the rest will be charged to the default charge card.