Fade to Black by Francis Knight

Mahala, a city built up not out, with a complex society run by a ruling elite from the bright upper levels while the darkness of the Under hides outlaws and forbidden secrets. Secrets such as the fact that bounty hunter/people finder Rojan Dizon is a pain mage and can draw magic from inflicting pain upon himself.

When Rojan's niece goes missing he must journey to the roots of the city, a world sealed off from the upper levels of the city, where he allies with fellow outcasts Pasha and Jake. Rojan's powers can help him find his niece, but he also must fight becoming addicted to using his power (which is why that magic is outlawed in Mahala).