Among the Hidden
Adventure; Dystopia; Young Adult
That's a whole lot of genre, so let's take this one at a time:
(1) The mystery of the face in the window sets up Luke for the adventure of his life. From the moment Luke sees the child's face, he is determined to see it again and find out who it is. Doesn't take too many games of Peeping Tom for Luke to come to the conclusion that it's another third child. If he didn't already plan on saving shadow children, he'd make one brilliant private detective.
(2) And though the mystery ends when he actually meets Jen, his adventure is just beginning. She introduces Luke to a world of good guys and bad guys, Davids and Goliaths, Matildas and Trunchbulls. And furthermore, Jen expects them to fight the good fight against the Population Law together. Luke hesitates at first, but all that changes after Jen dies and Luke takes the ultimate stand against the Government by setting a personal goal to bring justice to all shadow children (30.39).
(3) Speaking of the lovely Government, if they remind you of something you'd find in The Hunger Games or a Brave New World, it's because they all have the dystopia genre in common. Basically no one would have complains about heading off to Hogwarts or Narnia, but we're pretty sure that everyone would pass on shacking up in Panem or Luke's world. Dystopias all portray pretty cruddy places to live and they usually highlight an oppressive government which dictates how everything is run.
(4) We saved the easiest one for last. Protagonist between the ages of, say, 10 and 16? Check. Story of growth and maturation? Check. Sounds like a young adult novel to us.