It all depends.... If you look at the Booktype as collaborative interface where couple of people is editing a book, you shouldn't do direct insert on anything :)
But if you work with offline site or book that is not being edited by anyone that there is no reason why you couldn't insert directly into database. It is not recommended but you can create new chapter, you should write to history info also and possible update Table Of Contents. Command line or doing it from Django shell is much better options.