'Mortal Instruments' Prequel In The Works

The Mortal Instruments: City of Bones already has a prequel series in the works.  Stephanie Sanditz has been hired to adapt the Infernal Devices books.
Young adult franchise hopeful The Mortal Instruments: City of Bones doesn’t open until August 23, but Constantin Films is already working on a prequel.

The company has thrown into development Infernal Devices, an adaptation of the spin-off book series written by Mortal Instruments author Cassandra Clare, and has hired Stephanie Sanditz to write the screenplay.
Source: The Hollywood Reporter