Ghosts of the Shadow Market #1:Ghosts of the Shadow Market book by Cassandra Clare 

The Shadow Market is a meeting point for faeries, werewolves, warlocks and vampires. There the Downworlders buy and sell magical objects, make dark bargains, and whisper secrets they do not want the Nephilim to know. Through two centuries, however, there has been a frequent visitor to the Shadow Market from the City of Bones, the very heart of the Shadowhunters. As a Silent Brother, Brother Zachariah is sworn keeper of the laws and lore of the Nephilim. But once he was a Shadowhunter called Jem Carstairs, and his love, then and always, is the warlock Tessa Gray.