Valiant - Holly Black - paperback

Modern Faerie Tales, book 2 of 3, completed series

When seventeen-year-old Valerie runs away to New York City, she's trying to escape a life that has utterly betrayed her. Sporting a new identity, she takes up with a gang of squatters who live in the city's labyrinthine subway system. But there's something eerily beguiling about Val's new friends.

When one talks Val into tracking down the lair of a mysterious creature, Val finds herself torn between her affection for an honorable monster and her fear of what her new friends are becoming.