Linger by Maggie Stiefvater

What's on the back of the book:
I feel the weight of
the pack's gaze . . .

Grace and Sam must fight to be together. For Grace, this means defying her parents and keeping dangerous secrets. For Sam, it means grappling with his werewolf past . . . and figuring out a way to survive the future.

But just when they manage to find happiness, Grace realizes she's changing in ways she could never have expected . . .

A piece from the book:
This is the story of a boy who used to be a wolf and a girl who was becoming one.
Just a few months ago, it was Sam who was the mythical creature. His was the disease we couldn't cure. His was the goodbye that meant the most. He had the body that was a mystery, too strange and wonderful and terrifying to comprehend.
But now it is spring. With the heat, the remaining wolves will soon be falling out of their wolf pelts and back into their human bodies. Sam stays Sam, and Cole stays Cole, and it's only me who's not firmly in my own skin.
Last year, this was what I wanted. I had a lot of reasons to long to be part of the wolf pack that lives in the woods behind my house. But now, instead of me watching the wolves, waiting for one of them to come to me, they are the ones watching me, waiting for me to come to them.
Their eyes, human eyes in wolf skulls, remind me of water: the clear blue of water reflecting the spring sky, the brown of a brook churning with rainfall, the green of the lake in summer as the algae begins to bloom, the grey of a snow-choked river. It used to be only Sam's yellow eyes that watched me from between the rain-soaked birches, but now, I feel the weight of the entire pack's gaze. The weight of things known, things unsaid.
The wolves in the woods are strangers now that I know the secret of the pack. Beautiful, alluring - but strangers nonetheless. An unknown human past hides behind each pair of eyes; Sam is the only one I ever truly knew, and I have him beside me now. I want this, my hand in Sam's hand and his cheek resting against my neck.
But my body betrays me. Now I am the unknown, the unknowable.
This is a love story. I never knew there were so many kinds of love or that love could make people do so many different things.
I never knew there were so many different ways to say goodbye.