Among The Hidden by Margaret Haddix
First book in the shadow children sequence
This book, Among The Hidden by Margaret Haddix, Luke is a third child which is also known as shadow child. Later on Luke's favorite place is destroyed which is the woods so new fancy baron houses can be built. Now that there is new house Luke discovers another third child while peering out the window one day, Jen who wants to end the unlawful practice of third children. Now Luke spends everyday with Jen as many days go but Jen decides to fight for third children, but is considered a rebel and dies. Now the Luke is hiding from the population police and will soon be leaving.