A Light in the Dusk by Jessi Elliott, K.J. Sutton

Tunnels full of monsters. A half-vampire slave. A forbidden romance.
Charlie Travesty has been banished from the mansion.
Now bearing the tattoo that marks her as a slave, she begins work in the sewers, far below the city of New Ve. The weepers—vampires and humans alike who have been infected by a deadly virus—seem to be increasing in number, and the death toll is rising.
In order to ensure her survival, Charlie launches her own investigation. She finds an unexpected ally in infamous bounty hunter Noah Forrest, despite his ability to get under her skin like nobody else. With him at her side, and a human on the other, Charlie discovers that nothing is what it seems in the tunnels she now calls home.