Holly is the bestselling author and co-creator of The Spiderwick Chronicles series and won a Newbery Honor for her novel Doll Bones. Cassie is the author of bestselling YA series, including The Mortal Instruments, The Infernal Devices, and The Dark Artifices. They both live in Western Massachusetts, about ten minutes away from each other. This is the third book in Magisterium, following The Iron Trial and The Copper Gauntlet.