Thanks for the suggestion, we'll keep it in mind for the future. I don't know how much users would actually take advantage of a column like that but thanks for the idea in any case.
In the meantime you could use one of the custom fields.
May I suggest the "last read" field. You can then sort books in your read collection by the last read field. So after selecting the book and before dragging it to your read collection use the command-shift-T shortcut to mark it as read today. There is no need to even open the edit window. You will find other "mark as" options under the edit menu.
