Rushlight by L.R. Ryan

The future ... the remnant of mankind carries on; some are safe in an electronic citadel that keeps the elements at bay, while others have been driven underground into the old subway tunnels where the living is basic and hard-won. A young woman ventures from the safety of her underground tribe to search for food in the ruins of the old world, but finds a nightmare awaits her above ground as she’s captured and sold into slavery. She must find a way to escape, or hope for desperate rescue as a rebellion builds in the labyrinth of tunnels beneath the ruins.