Healing Wars, book 1: The Shifter / The Pain MerchantsJanice Hardy2009

Nya is an orphan struggling for survival in a city crippled by war. She is also a Taker - with her touch, she can heal injuries, pulling pain from another person into her own body. But unlike her sister, Tali, and the other Takers who become Healers' League apprentices, Nya's skill is flawed: She can't push that pain into pynvium, the enchanted metal used to store it. All she can do is shift it into another person, a dangerous skill that she must keep hidden from forces occupying her city. If discovered, she'd be used as a human weapon against her own people.

Rumors of another war make Nya's life harder, forcing her to take desperate risks just to find work and food. She pushes her luck too far and exposes her secret to a pain merchant eager to use her shifting ability for his own sinister purposes. At first Nya refuses, but when Tali and other League Healers mysteriously disappear, she's faced with some difficult choices. As her father used to say, principles are a bargain at any price; but how many will Nya have to sell to get Tali back alive?