Freedom's Apprentice — Naomi Kritzer

Once the trusted aide to powerful military commander Kyros, freeborn Lauria hunted down his escaped slaves. But during a mission to infiltrate the bandit tribe known as the Alashi, Lauria's loyalties shifted. When her identity was discovered, she was cast out by both sides. Now Lauria is determined to regain the trust of the Alashi, and, with the help of her blood-sister Tamar, liberate those she once returned to captivity. But they cannot accomplish the daunting task alone.

Desperate for a spell-chain to free a mine slave, Lauria turns to her enemies -- the Sisterhood of Weavers -- and apprentices herself to a sorceress. But learning to harness magic will come at a greater price than she ever imagined.