After looking over the list of the books I read in October - my top pick is: The Replacement by Brenna Yovanoff
I love changeling type stories; The Stolen Child by Keith Donohue could almost be the companion piece to this one.
(My favorite is Stolen by Vivian Vande Velde)
The interesting thing about The Replacement is the family and Mackie know he is a replacement and the story relates to how they adjust and what it means to be human.
Summary by Goodreads:
Mackie Doyle is not one of us. Though he lives in the small town of Gentry, he comes from a world of tunnels and black murky water, a world of living dead girls ruled by a little tattooed princess. He is a Replacement, left in the crib of a human baby sixteen years ago. Now, because of fatal allergies to iron, blood, and consecrated ground, Mackie is fighting to survive in the human world.
Mackie would give anything to live among us, to practice on his bass or spend time with his crush, Tate. But when Tate's baby sister goes missing, Mackie is drawn irrevocably into the underworld of Gentry, known as Mayhem. He must face the dark creatures of the Slag Heaps and find his rightful place, in our world, or theirs.