Envy and Magic (Spells and Sins Book 2) by Melody Raven
Trying to keep her mind off the mess of her personal life, Sam Harris throws herself into the training she could never get a handle on as a teen. Maybe if she works hard enough, she can even become a witch who, you know, can actually use her powers. But nothing is free, and if she wants to train, she has to become a full fledged member of the Families. But when she's assigned one of her first tasks for the Families, to find out who is burning the homes of local witches, she's once again confronted with the man she'd hoped to never see again: Detective Derek Pierce. Derek is determined to figure out what the witch community has to do with the fires before anyone gets hurt. Anyone human at least. His investigation hits a brick wall when Sam and her family get in his way. Now the only way to find the answers is to work with the one woman he never wanted in the middle of this mess. Except the person behind the fires is closer than any of them realize. And the closer they get, the closer they become to being the next targets.