OK, the serious post: The size of books vs shelves messes things up a bit. I have one nice wooden bookshelf which is old and can ONLY hold small books. So it has 50s paperback histories (E.g. Horrock's autobiography) etc. I have managed to broadly separate my books by theatre. I haven't got enough books on the war at sea or in the air to need to subdivide those. Tanks as tech go in a separate category.