Tatter and Shine - Troemner J W

For years Shine has studied magic in secret, buying black-market spellbooks and bribing two-bit warlocks for their scraps of knowledge, but every spell she learns comes at a terrible risk. In a land ravaged by war with the Sorcerer Queen, magic is outlawed, and those caught practicing it are hunted and killed. 

When the lessons run out and the witch hunt closes in, Shine seeks out a new mentor: the immortal sorcerer Tatterdemalion, maker of deals and master of demons. The closer she gets to her new tutor, the more she uncovers the mysteries of his past, the source of his power, and his part in the war that changed the world.

But Shine’s rise doesn't go unnoticed. The Sorcerer Queen has her eyes on Tatterdemalion’s new apprentice, and she will not tolerate a rival to her power.