Book Tracker for iOS (it includes iPhone and iPad) and Mac are two separate apps. Developing and maintaining each version takes a lot of hard work and a lot of time.
In order to ensure constant updates and new features and to be sustainable as an independent developer, two separate purchases are required. Note that this is not a subscription but a single purchase for the iOS app and one for the macOS app. Despite being two separate apps, the library automatically syncs between iOS and Mac via iCloud.