The Missing Book 1: Found by Margaret Peterson Haddix

Jonah and Chip receive mysterious letters telling them they are one of the missing. With the help of Jonah's sister, Katherine, they go on an adventure to find out who they really are.
~Amica, age 12

Book Description: When 13-year-olds Jonah and Chip, who are both adopted, learn they were discovered on a plane that appeared out of nowhere, full of babies with no adults on board, they realize that they have uncovered a mystery involving time travel and two opposing forces, each trying to repair the fabric of time.