Shiver - Maggie Stiefvater

Grace is fascinated by the wolves in the woods behind her house;
one yellow-eyed wolf in particular. Every winter, she watches him,
but every summer, he disappears.
Sam leads two lives. In winter, he stays in the frozen woods, with
the protection of the pack. In summer, he has a few precious months
to be human . . . until the cold makes him shift back again.
When Grace and Sam finally meet, they realize they can't bear to
be apart. But as winter nears, Sam must fight to stay human - or
risk losing himself, and Grace, for ever.