City of Bones by Cassandra Clare

It's a good thing vampires and werewolves and fairies are mythical because every author who uses them in books can use them how they want.
This book has a different name for those who hunt down the demons: Shadowhunters. Ordinary humans or 'mundanes' can't see the Shadowhunters or the ones they hunt. Clary can't see them either until the night she witnesses a murder. Suddenly, another world opens to her vision and she is thrust into danger.

It was interesting enough to keep me reading, trying to figure out the truth.
And that's all I have to say about that :)

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. Then the body disappears into thin air. It's hard to call the police when the murderers are invisible to everyone else and when there is nothing - not even a smear of blood - to show that a boy has died. Or was he a boy? This is Clary's first meeting with the Shadowhunters, warriors dedicated to ridding the earth of demons. It's also her first encounter with Jace, a Shadowhunter who looks a little like an angel and acts a lot like a jerk. Within twenty-four hours Clary is pulled into Jace's world with a vengeance, when her mother disappears and Clary herself is attacked by a 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....