Daybreak by Ellen Connor

Their mission was to save the world.
Their destiny is to fall in love.

Twelve years since the Change, Penelope Sheehan is one of the few still practicing magic for the good of humanity. To infiltrate the notorious O’Malley organization, Pen poses as an abducted girl—until a furious lion thwarts her mission. When the beast turns into a devastatingly handsome man, she recognizes in him the troubled boy she once knew.
Since becoming a skinwalker, Tru Daugherty has allowed his animal nature to take over. Aloof and cynical, he’s a creature of instinct and impulse, living only to satisfy his senses—ignoring the scarred heart nobody has ever reached. He’s also the best man to help Pen bring down the O’Malley crime ring.
Fighting alongside the last holdouts of humanity, they unleash a passion that tempts them to risk everything for love. But if they succeed, Tru and Pen hold the power to brighten the Dark Age for all time.