The Secret Adversary by Agatha Christie

Another great Christie thriller, and yet another with non-familiar characters (although Inspector Japp is mentioned a couple of times). Tommy and Tuppence are Young Adventurers, and what an adventure they get caught up in. Kept me guessing right to the end - every time it seemed to be one person, I decided it was the other, and kept on changing my mind with each new clue or revelation. Fun!