Mage to Disobey by Mandy Rosko

Lisa Chen has finally made a decision.
She’s going to give up her life of magic and settle for a non-magic life with her human ex.
Well, mostly non-magic. She’d never abandon Angus , her little red dragon familiar.
But no sooner does Lisa reconcile with Jason Cross when a strange symbol appears on her arm. What does it mean? She’ll have to re-enter the magic world to find out, which means leaving Jason yet again.
She promises it will be the last time, but promises are shattered when she uncovers the meaning of the symbol: She’s been chosen to join the world’s twelve most powerful mages on The Nightshade Guild.
She’s shocked. Especially when the other Nightshade Guild members tell her that she needs to return with them and take the oath.
Why did the symbol chose her? She’s never been good at playing by rules, and now she’d be expected to be one of the rule keepers. An impartial protector and enforce of rules regarding the all of the world’s magical creatures.
She’s sure she can’t do it, because Lisa was Mage to Disobey.