Every number counts indeed, at least when you are looking for books on our top floor! For example, you want a book with the call number HG4921. You go upstairs, see a block of shelving that starts with HC, work your way to the end, where the run of books ends with the HG4920s. That can be a little disconcerting, but all you need to do is to walk back up and over to the next block of shelving and there you will find the HG4921s!
There are floor plans of the library here, including of the top floor, and you are always welcome to ask one of us for help.