TODO:
    * book.parent is an array, not a dict - should change it over

    * search on more than title, e.g. author, series, etc.

    * if we add a new parent book with sub-books into a series, with parent * book as 'null' sub_book_num, all the sub_books should be added somehow into series - but cant input numbering on parent book, so
        - don't allow adding parent, use a pop-up saying, add the first-child, and then auto-number from there

    * consider removing all direct sqls and using ORM properly:
        - grep 'with db.engine' *.py