Traitor - Mark Eller

Traitor is the first novel in the Turner Chronicles series. This book is 99 cents for the kindle.

In a parallel universe magic is not only possible. It's common. But to really use your magic you need a magic stone. Aaron Turner is one person who can travel between realms. He's a cripple in his home world, where he's forced to help an ambitious militia general. In the target realm Aaron owns a simple general store and will eventually change the world. Very highly recommended.