The book discount is part of the subscription process. Go to https://touringplans.com/join. Click on the park icon you want and on the next page, you will see "Own a current Unofficial Guide." Click on the link and then click on the dropdown arrow and select the book you purchased. The system will ask you questions to validate your purchase. You can also add another subscription on the last page.
Apple and Google Play do not offer the book discount.