I'd like to support this request. I was frankly quite surprised to see that it isn't already in there, considering how full featured Comix is in most other respects.

I did some hacking on this, and while simply adding a <read> field to the database and updating that whenever the user reached the last page of a comic was quite easy, what has turned out to be really tricky is how to display this in the library. I was thinking of overlaying an emblem just like in Nautilus, but I haven't found a way to do this yet.