Once you have clicked the link from your email you should be presented some options (depending on your iPhone and version of iOS). Screenshots are of iOS 14.
Scroll far to the right past all your apps to find ... More (see below):
Once you have clicked more you'll see a list of your apps. You'll want to find the native iOS app called BOOKS.
That's it. You can now reference your digital downloads on any/all of your iOS devices.