To delete a custom attribute from a single book, simple find the book and click the trashcan icon next to the attribute.
To remove a custom attribute entirely from the system, follow these steps:
- Click the Search box at the top of the Books tab.
- Click the "Add an attribute to these X books" button.
- Select the attribute you'd like to delete on the left, and then select "Not set" on the right.
This will unset the attribute for every book in your account, which removes it entirely.