Missing Collection by Margaret Peterson Haddix

Thirteen-year-old Jonah has always known that he was adopted, and he''s never thought it was a big deal. Then he and a neighbor, Chip, who finds out he''s also adopted, begin receiving mysterious letters, saying things such as: "You are one of the missing," and, "Beware! They''re coming back to get you."

Jonah, Chip, and Jonah''s little sister Katherine are plunged into a mystery that involves the...