Shiver (The Wolves of Mercy Falls, #1) by Maggie Stiefvater

the cold.

Grace has spent years watching the wolves in the woods behind her house. One yellow-eyed wolf - her wolf - watches back. He feels deeply familiar to her, but she doesn't know why.

the heat.
Sam has lived two lives. As a wolf, he keeps the silent company of the girl he loves. And then, for a short time each year, he is human, never daring to talk to Grace... until now.

the shiver.
For Grace and Sam, love has always been kept at a distance. But once it's spoken, it cannot be denied. Sam must fight to stay human - and Grace must fight to keep him - even if it means taking on the scars of the past, the fragility of the present, and the impossibility of the future.