Valiant by Holly Black

From the publisher (source: The Internet Book List):
When seventeen-year-old Valerie Russell 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. Impulsive Lolli talks of monsters in the subway tunnels they call home and shoots up a shimmery amber-colored powder that makes the shadows around her dance. Severe Luis claims he can make deals with creatures that no one else can see. And then there's Luis's brother, timid and sensitive Dave, who makes the mistake of letting Val tag along as he makes a delivery to a woman who turns out to have goat hooves instead of feet.

When a bewildered Val allows Lolli to talk her into tracking down the hidden lair of the creature for whom Luis and Dave have been dealing, Val finds herself bound into service by a troll named Ravus. He is as hideous as he is honorable. And as Val grows to know him, she finds herself torn between her affection for an honorable monster and her fear of what her new friends are becoming.

This book is part of the "Modern Faerie Tales" series.
Here are some other books from this series: