Shiver (Scholastic, 2009) by Maggie Stiefvater
Grace was attacked by the wolves when she was small, but "her" wolf saved her from the others.
When it gets cold Sam is a wolf. He's been staying a wolf longer and longer and this may be the last time he's a human.
The story is told from both Grace's and Sam's viewpoints.
You don't want to miss this story of love, betrayal, and reconciliation. Neither do you want to miss the book trailer on Maggie's site.
And wait! A sequel, Linger, is coming this summer! A third book, Forever, is also planned.