User blog comment:Add799/Nine demon gates a type of celestial spirit magic?/@comment-7418318-20131028153432

For now the common theory is, that each Demon came from one book, so rather than them turning to keys, they would return into their book.

Giving those books over into the care of the Celestial Spirit King seems like a good idea. There are very few Celestial Spirit Mages left in the world and juding from the Kings words, Lucy was the first one ever admitted into the Celestial Spirit World, so it'd be a very save place to store them, if they can't or shouldn't be destroyed for some reason.