It's cool to be able to compared two custom tags, but I'd like to be able to compare multiple and filter by year, shelf or other criteria.

As an example case, I like to try and keep track of where the authors are from for each book I read and see what country produced the most books I read at the end of the year. The current custom tags make it a bit tedious to see this tag info, and have no way of filtering it by when I read a given book, for example.