The Magicians' Guild (Black Magician Trilogy)

Caught up in a confrontation between the hated Guild magicians and the angry youth of the slums Sonea discovers, to her dismay, that she possesses the same powers as the reviled magicians. To avoid capture, she must seek protection from the city's feared underworld, The Thieves. In return she must use her powers for them, but her magic never seems to do quite what she intends it to...