Shadow Gambit by A.S. Green

Turns out all’s NOT fair in love and war, and this supernatural battle is just beginning.
Stella and Ethan are quickly learning that every step forward leaves them two steps back. Sure, they now know where to find the countercurse that will give them a shot at happily ever after, but stealing another coven’s grimoire was never going to be easy.
And even if they can get their hands on the spell book, there’s no guarantee they haven’t been tricked.
Their last hope ends up being their next mistake. Breaking Bridget Bishop’s centuries-old curse on Ethan’s family line causes a new kind of evil to leak into the world—an evil that has them fighting for their lives against an enemy neither of them foresaw.