Note that the book discount is only available for purchases made via our website. There are no discounts whatsoever for in-app purchases (Apple and Google Play) from our Lines apps , so do not expect a book discount there.
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 an "Own a current Unofficial Guide?" link. Click on that link and then select the book version you purchased. The system will ask you questions to validate that you own the book. You can also "add-on" another subscription, too, but note that there is only one book discount per purchase, no matter how many books you own.