Shiver by Maggie Stiefvater is the first book in the Wolves Of Mercy Falls trilogy.
Shiver is about Grace and Sam.
When the temperature slips towards freezing, Sam reverts to his wolf identity and must retreat into the woods to protect his pack. He worries that eventually his human side will fade away and he will left howling alone at the lonely moon.
I really enjoyed reading Shiver by Maggie Stiefvater! My favourite character was Sam, because of his optimism.
Available at Amazon.co.uk.