I just stumbled upon your plugin today. Very cool.
It would be very useful to me if the ability to add multiple custom fields beyond just the Custom Links option. For example, books at the elementary ed level are usually assigned a Reading Level by either the book’s publisher. It would be really nice to be able to add this information alongside all of the other book data being pulled from the Google API.
Version 2.3.0 of the Book Review plugin was just released that adds support for custom fields. You’ll see a new Custom Fields tab in the Book Review Settings. Just enter the custom fields that you want on this tab, and you’ll then be able to enter values for each one at the post level.
Please note that custom fields will not be populated by the data in Google Books since there’s no way to map custom fields to data in Google Books, if it even exists there at all.