Ironside : a modern faerie tale
Holly Black
When Valerie runs away to New York to escape her old life, she decides to sport a new identity and take up with a gang of squatters who live in the city's labyrinthine subway system. There's something suspicious about her new friends, but she can't quite put her finger on it. When Val is talked into tracking down the lair of a mysterious creature, she's drawn into a world she never knew existed.