WILTWTR4TSP: Among the betrayed by Margaret Peterson Haddix | Y.O!

Found - Margaret Peterson Haddix. I thought this Oregon Battle of the Books title was a very good page turner. It's a somewhat plausible premise at first, but it requires a little more suspension of disbelief as it goes on.
Summary from Goodreads: Thirteen-year-old Jonah has always known that he was adopted, and he's never thought it was any big deal. Then he and a new friend, Chip, who's also adoped, begin receiving mysterious letters. The first one says,