Forever By Maggie Stiefvater

then.
When Sam met Grace, he was a wolf and she was a girl. Eventually he found a way to become a boy, and their love moved from curious distance to the intense closeness of shared lives.

now.
That should have been the end of their story. But Grace was not meant to stay human. Now she is the wolf. And the wolves of Mercy Falls are about to be killed in one final, spectacular hunt.

forever.
Sam would do anything for Grace. But can one boy and one love really change a hostile, predatory world? The past, the present, and the future are about to collide in one pure moment–a moment of death or life, farewell or forever.