Cassandra Clare has teen fiction down like 'Clockwork'
A link has been posted to your Facebook feed. A tweet has been posted to your Twitter account. A link has been sent to your friend's email address. There's no "magic formula" for a successful YA book, says Cassandra Clare. Here, Lily Collins and Jamie Campbell Bower star in a scene from the movie adaptation of her book 'The Mortal Instruments: City of Bones.' (Photo: Rafy Screen Gems)