Whats the benefit of Templates if you can’t use them with a different book size. It seems like Duplicate Book and Create Template from Book do the exact same thing.
I thibk each size has it’s OWN template and can be used as template for creating more books of the same size. Duplicate would create a file full of "stuff" to be removed when you do the "new book" – easier to start with a blank template.
I don’t know exactly what you mean, but I think it’s quite normal that every book size has its own templates. I make my books from scratch, design my own templates and save them as such.