Most young-adult paranormal romancey novels use one narrator, usually a female, but sometimes a male. In Shiver, you get both. Maggie Stiefvater effortlessly shifts between Grace's female perspective and Sam's male perspective. Sometimes Sam's a wolf, so it's almost like you get 2.5 perspectives for the price of one. Bargain!
So what does this do for the story? You know how you and your friend can have different points of view about the exact same thing? She says she borrowed your Adventure Time hat, you say she stole it... Well, seeing things through Sam's and Grace's eyes allows us to see the same events through two different perspectives. And guess what? Both of them are valid. Switching between Sam and Grace shows us that it's not so much what happens that matters, but what you make of it.