I'm looking to find more examples of books which bridge the gap between "story" and "mathematics" using narrative and all those other wonderful features we might find in Harry Potter or some other well known children's book.
So far, Knuth's Surreal Numbers is one of the few attempts I've seen from a professional mathematician to produce a work of fiction which is also a work of serious mathematics: http://www-cs-faculty.stanford.edu/~uno/sn.html
Any other similar books would be a good place to look when thinking about "how do I sell this mathematical idea to kids"?