A "you've read this" would be nice. As my collection increases in becomes more difficult for me to remember which comics I've read and which I haven't. So perhaps a little icon graphic that appears on top of the covers in My Collection, or maybe the ability to list by 'read' and 'not read'.
Making this idea work similar the way Amazon uses "I Own This" could also be used to add a greater ability to suggest books Users could find interesting or similar to the books already they currently own. (provided that rating and submitting reviews would also be allowed once the User marks a comic as "Owned")