This is the first book in a series by Margaret Peterson Haddix. Jonah and his friend Chip (both adopted) start receiving vaguely threatening letters that seem to imply that they were stolen children. The boys and Jonah's younger sister embark on an adventure matching wits with rogue FBI agents, time-traveling mystery men, and other "missing" children.
What Haddix does well is come up with suspenseful adventure series for middle school kids. What this book lacks is depth of character, timelessness, and any type of timeless truth. Big nay from me.