There's an entertaining discussion, begun here by Sarah Crown, on the best way of arranging your books: by subject, by colour, by publisher, alphabetically, in the order you've read them, any old how. The beauty of it is that there's no optimal solution. If you do it by subject or genre, you soon discover books that belong in more than one place, and also books that fit somewhere in between two subjects. Can you organize things so that those two subjects are spatially adjacent, with the in-between books located in between them? Maybe. But you then find that you can't have all the subjects adjacent that need to be in order to accommodate all the in-between books. If you go with alphabetical ordering, some enormous books have to sit next to some tiny ones, and that looks ugly. And so on.
I'm sure there must be a formal proof showing that every system for arranging books is imperfect.