So, I've decided to do a double-marking sort of a thing to keep track of my bingo progress.
First, I need a way to mark when a square has been called. I decided to do a red circle to delineate once a square gets called. Second, I need a way to mark when I've finished the book for the square - so I decided to use a skull and cross-bones icon that fits mostly inside the red circle to track this. So, my card will look like this:
This shows that the squares "serial/spree killer" and "murder most foul" have both been called and I've finished books for "murder most foul" and "genre: horror." The completed square is "murder most foul" because it has both the red circle and the skull and cross-bones to show it's been called and read.