FR: filter and categorize books with tags, subjects, types, book statuses

Please add ability for books to be placed into more than one group at a time.

I have upwards of 9,000 books in my library and plan to use the desktop app as a "library manager", basically load all of my books into the desktop app and manually feed them to mobile devices from there. I currently have them grouped by author/series, then also have groups for "currently reading", "on deck", "wife's books", etc.

Currently if I move a book to the "currently reading" group it removes it from the author/series group it was assigned to - books can't be in multiple groups at a time. I'm working around this by manually moving books back to their author/series group after they're completed, but it would be better if they could reside in multiple groups simultaneously.