Valiant

After discovering her mother's affair, seventeen-year-old Valerie Russell runs away to New York City. There she falls in with a group of teen squatters who live in an abandoned subway tunnel and who deliver a magical drug called Never. Val, who takes the nickname Prince Valiant, is drawn into the city's hidden faerie underworld, where she meets Ravus, a troll who lives beneath a bridge and who teaches her swordplay. She is torn between her affection for an honourable monster and her fear of what her new friends are becoming.

Valiant is the second book in Holly Black's Modern Tales of Faerie, following Tithe, and it can be read on its own. It won the first Andre Norton Award for young adult science fiction and fantasy.