I just put them on the shelf as they come in. I don't keep a lot of books so it's not diffcult to keep track of them. I know most books i won't read again, so i keep the ones i like for display, such as my favorites, really old looking books (coz i love the look and smell of them), collectors books, and books i know i want to read again at some point. Overall i don't really keep a lot of books. The rest just go to the charity shop.