Shiver by Maggie Stiefvater

For years, Grace has watched the wolves in the woods behind her house. One yellow-eyed wolf—her wolf—is a chilling presence she can't seem to live without.

Meanwhile, Sam has lived two lives: In winter, the frozen woods, the protection of the pack, and the silent company of a fearless girl. In summer, a few precious months of being human… until the cold makes him shift back again.

Now, Grace meets a yellow-eyed boy whose familiarity takes her breath away. It's her wolf. It has to be. But as winter nears, Sam must fight to stay human—or risk losing himself, and Grace, forever.