I've often pondered the optimal book organisation, sorting and inserting system. Organising by subject seems a good start, but then you have size to conser. Then you also tend to arrange by space - i.e. you might put two subjects together based more on similar size or shelf capacity then logical subject grouping.

For inserts a reverse-sorted inserting-from-the-back makes most sense to reduce moving of books, but I suspect there's probably an optimal 'free space' value to allow for easier inserting.