Cherry Bomb by Caitlin R. Kiernan

Meet Siobhan Quinn-Half vampire, half werewolf, and retired monster hunter. Or so she thought.
Three years have passed since Quinn turned her back on Providence, Rhode Island's seedy supernatural underbelly, walking out on Mr. B. and taking a bus headed anywhere. She hoped her escape would give her some peace from the endless parade of horrors. But a dead girl who quarrels with the moon can't catch a break, and, on the streets of Manhattan, Quinn finds herself caught between a rock and a hard place. Again.
What do you do when you're stuck in the middle of a three-million-year-old grudge match between the ghouls and the djinn, accidentally in possession of a hellish artifact that could turn the tide of the war, all the while being hunted by depraved half-ghoul twins intent on taking the object and ushering in a terrifying Dark Age?
Especially when you've fallen in love with the woman who got you into this mess-and you ain't nobody's hero.