Clockwork Angels (Infernal Devices #1) by Cassandra Clare Book Trailer:

Magic is dangerous- but love is more dangerous still.

When sixteen-year-old Tessa Gray crosses to find her brother, her destination is England, the time in the reign of Queen Victoria, and somehting terrifying is waiting for her in London's Downworld, where vampires, warlocks and other supernatural flock stalk the gaslit streets. Only the Shadowhunters, warroirs dedicated to ridding the world of demons, keep order amidst the choas.

Kidnapped by the mysterious Dark Sisters, members of a secret organization called the Pandemonium Club, Tessa soon learns that she herself is a Downworlder with a rare ability: the power to transform, at will, into another person. What's more, the Magister, the shadowy figure who runs the Club, will stop at nothing to claim Tessa's power for his own.

Friendless and hunted, Tessa takes refuge with the Shadowhunters of the London Institute, who swear to find her brother if she will use her power to help them. She soon finds herself fascinated by-and torn between-two best friends: James, who's fragile beauty hides a deadly secret, and blue-eyed Will, whose caustic wit volatile moods keep everyone in his life at arms length... everyone, that is, but Tessa. As their search draws deep into the heart of an arcane plot that threatens to destroy The Shadowhunters, Tessa realizes that she may need to choose between saving her brother and helping her new friends save the world...and that love may be the most dangerous magic of all.