Shatterglass. Tamora Pierce. 2003.

Tris is one of the most powerful mages ever born, her lightning magic flows through her veins and bones. When a series of brutal stranglings desecrate the fragile peace of Tharios, Tris is infuriated by the attitude of the city-fathers. No one seems to care about the dead girls. But Tris knows she has to keep hold of her magic she can’t let anger take over.

So when she feels a mage pulling all the power in the neighbourhood into his working, Tris is deeply disturbed. That mage, Keith, is completely unaware of his magic. Left a shell of his former self after he was struck by lightning; he thinks he’s useless. Tris is sure his magic could help her to find the killer, but can she and Keth unravel his powers in time?