Personally I wouldn't divide by either subject or genre since there will be a lot of books that will fit in several groups and you have to decide now which group to put them in and next year you'll have to remember which group you assigned them to. You could avoid that problem by putting each book in as many groups as it might fit but then, after you read the book, you might decide it belongs somewhere else. Either way you have a management nightmare or a sloppy system.
What I think I'd do is something like put books who's primary author's last name falls between A and E on card 1, F to I on card 2 and so on, of course first counting how many authors last names begin with each letter to determine the proper mix.
Then, a year or two from now you'll always know exactly where each book is. No management problems and no confusion.
I do realize that seems less artsie. But it's a heck of a lot more practical.
It doesn't give you the satisfaction of having done something amazing. It just simply works. What fun is that?
Barry