I understand the sorts available. If you use page a number as part of title it can sort page by page.If you want to add a page in the middle of you "book" the page would have to be 9a to fit in. If you wanted to a page between 9 and 9a it become cumbersome. If the book went over 99 pages (unlikely but can happen) all the pages would have to be re-numbered 001 unless you did that in the beginning. If EN wants to compete with ON this is one thing to add. I use ON for books and EN for tagging but I could get rid of ON with this feature.