City of Bones: Mortal Instruments, Book 1 (The Mortal Instruments)

Their hidden world is about to be revealed....
When fifteen-year-old Clary Fray heads out to the Pandemonium Club in New York City, she hardly expects to witness a murder -- much less a murder committed by three teenagers covered with strange tattoos and brandishing bizarre weapons. Clary knows she should call the police, but it's hard to explain a murder when the body disappears into thin air and the murderers are invisible to everyone but Clary.
Equally startled by her ability to see them, the murderers explain themselves as Shadowhunters: a secret tribe of warriors dedicated to ridding the earth of demons. Within twenty-four hours, Clary's mother disappears and Clary herself is almost killed by a grotesque demon. But why would demons be interested in ordinary mundanes like Clary and her mother? And how did Clary suddenly get the Sight? The Shadowhunters would like to know....