Suggestions based on your edit:
Peter Singer, The Life You Can Save
Ursula LeGuin, The Ones Who Walk Away from Omelas
Katherine Boo, Behind the Beautiful Forevers
Albert Camus, The Stranger and The Myth of Sisyphus (preferably read together, since the latter can really help illuminate the former)
Suggestions based on your edit:
Peter Singer, The Life You Can Save
Ursula LeGuin, The Ones Who Walk Away from Omelas
Katherine Boo, Behind the Beautiful Forevers
Albert Camus, The Stranger and The Myth of Sisyphus (preferably read together, since the latter can really help illuminate the former)