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 your name in the lower-left corner, or choose Account > View My Account.
Scroll down, click Redeem Gift Card or Code, click OK to open the app store, 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, see the Apple Support article Check your Apple ID balance.
Gift cards are valid only in the country or region where they were purchased.