The Secret Adversary (Tommy & Tuppence Mysteries)

The girl in the photograph has been missing for five years. Neither her body nor the secret documents she was carrying have ever been found. Now, post-war England's economic recovery depends on finding her and getting the papers back. But the two young Brits working undercover for the ministry know only that her name was Jane Finn, and the only photo of her is in the hands of her rich American cousin. They don't know yet about a mysterious and ruthless man called "Mr. Brown," or the beautiful but sinister older woman who knows all about Jane Finn...and therefore must die.