Shiver by Maggie Stiefvater

Shiver is the first book in the triology The Wolves of Mercy Falls by Maggie Stiefvater. Shiver was released 1 August 2009, followed by Linger on 13 July 2010 and Forever, released 12 July 2011.
The books The Wolves of Mercy Falls follow Grace Brisbane and Sam Roth. Shiver is written from both Grace and Sam's point of view. Linger is written from Grace, Sam, Isabel and Cole's view point. Forever is written from Grace, Sam, Isabel and Cole's point of view, with a small prologue from Shelby. The book series The Wolves of Mercy Falls is based in the fictional town Mercy Falls, Maggie Stiefvater has said that the real town of Ely in Minnesota would be the closest in destination to Mercy Falls. Grace has watched the wolves in the woods behind her house. One yellow-eyed wolf—her wolf—is a chilling presence she cannot 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.
Then, Grace meets a yellow-eyed boy whose familiarity takes her breath away. It's her wolf. It has to be. Grace hides Sam in her house and eventually, Sam becomes her boyfriend. But as winter nears, Sam must fight to stay human—or risk losing himself, and Grace, forever.
Shiver has been licensed in over 32 foreign territories and remained on the bestseller list for over 32 weeks, selling 130,996 copies in 2009. (Summary by Wikipedia)