Tarnished (The St. Croix Chronicles) by Karina Cooper - Romance>Fantasy
My name is Cherry St. Croix. Society would claim that I am a well-heeled miss with an unfortunate familial reputation. They've no idea of the truth of it. In my secret world, I hunt down vagrants, thieves . . . and now, a murderer. For a monster stalks London's streets, leaving a trail of mystery and murder below the fog.
Eager for coin to fuel my infatuations, I must decide where my attentions will turn: to my daylight world, where my scientific mind sets me apart from respectable Society, or to the compelling domain of London below. Each has a man who has claimed my time as his—for good or for ill. Though as the corpses pile, and the treacherous waters of Society gossip churn, I am learning that each also has its dangers. One choice will see me cast from polite company . . . the other might just see me dead.