Redeem an Apple Gift Card in Books on Mac
If you received an Apple Gift Card, iTunes Store Gift Card, or promotional code, you can use it to buy books, audiobooks, and other items in the Book Store or Audiobook Store.
Click Featured (below Book Store or Audiobook Store) in the sidebar.
On the right below Quick Links, click Redeem, then follow the onscreen instructions
You don’t need to enter credit card information to redeem only the amount of a gift card.
To check your balance, choose Store > View My Apple ID, sign in (if necessary), then go to the Apple Account section.
Gift cards are valid only in the country or region where they were purchased.