Here's the amazon.com description:
When sixteen-year-old Tessa Gray crosses the ocean to find her brother, her destination is England, the time is the reign of Queen Victoria, and something terrifying is waiting for her in London's Downworld, where vampires, warlocks, and other supernatural folk stalk the gaslit streets. Only the Shadowhunters, warriors dedicated to ridding the world of demons, keep order amidst the chaos.Much of the book is written from Tessa's point of view. Because she starts the book ignorant of the supernatural world - she is in London only because she is looking for her brother - we learn about the workings of this world along with Tessa. This has the advantage of giving the reader a slow introduction to the different factions - Shadowhunters and Downworlders - and their powers.
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, whose fragile beauty hides a deadly secret, and blue-eyed Will, whose caustic wit and volatile moods keep everyone in his life at arm's length... everyone, that is, but Tessa. As their search draws them 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.
Like many young adult novels, this book is long on violence, long on budding romance, and short on sexual situations. One of the oddities of the genre is that an adult level of violence is considered more acceptable than an adult level of sex. Tessa is drawn to both boys, James and Will, in different ways, and they are both interested in her. Presumably the other books in the trilogy will explore these relationships further.
Because this is the first book of a trilogy, the books leaves a number of loose ends. While the book does resolves some of the plot lines, it's a little unsatisfactory as a stand-alone novel. However, as the first part of a longer work, it provides an intriguing setup.
The second book in the series, Clockwork Prince, is in publication, while the final book, Clockwork Princess, is due in 2013.