Haddix, Margaret Peterson (2008). Found. (The Missing, bk.1) 314 pages. Grades 5-7.
Thirteen-year-old Jonah always knew that he was adopted. His parents were honest with him from the beginning. His friend, Chip, didn't learn that he was adopted until more recently. And it came as a complete shock. Unlike Jonah's parents, Chip's parents do not want to discuss the details with him and would rather he not ask them any questions. But, when both boys begin receiving anonymous letters concerning their adoptions, they begin to think that something is not right. With the help of Jonah's sister, Katherine, they attempt to figure out who is sending the letters and why. In the process, they discover that as babies, they were found on a plane that appeared out of nowhere, with no adults on board. Now it is up to the three friends to warn the other people who were on that plane before it is too late.