Don't Savage The Messenger
There is an uneasy truce between the werewolves in the woods and the humans who live in Beacon Hills, protected by a magical boundary that gives warning any time a werewolf crosses it. Then the sheriff is taken by the werewolves and his son offers himself in exchange.
Stiles promises to serve the werewolf pack, not knowing what horrible use they might have for him. But it turns out his most useful skill is the ability to cross the boundary line between humans and werewolves. Life with the werewolves is nothing like he feared and the werewolves themselves are nothing like the hunters' stories would have him believe.