[roryokane]

Thanks for the method. I’ve written up detailed instructions for it at https://apple.stackexchange.com/a/382603/21473.

As you can see in that answer, I found a better method to assign the keyboard shortcut. Assigning the shortcut within App Shortcuts instead of Services lets you use the normal ⌘C shortcut in Books while not affecting copying in other apps.

[stassats]

Limiting the action to "Books" seems to not affect other apps as well. The main reason I did not use ⌘C is because it doesn't work when the context menu is displayed (and it gets displayed immediately after selecting text).