The Reach Between Worlds by Cameron Hayden

Posing as a sorcerer was never a smart idea. Now, it may cost Taro his life. Sixteen-year-old Taro lost his right leg years ago, but with a family to support, he doesn't let it slow him down. For years he's worked for a notorious crime lord named Victor Mathan; Mathan deals in stolen magical artifacts, and trusts Taro to finish his work with as few questions as possible. When an absurdly well-paying job comes up, Taro is quick to volunteer. Mathan wants him to break a powerful sorceress out of her magical prison, but her cell is under the control of an ancient military order called the Magisterium. To get to her, Taro must pose as a Magisterium recruit. The courses are grueling, the tests drive students mad, and the magic they teach can be as deadly to the caster as it is to the target.