Secret Adversary by Agatha Christie, Fiction, Mystery & Detective

The Great War is over, and jobs are scarce. Tommy Beresford and Prudence "Tuppence" Cowley meet and agree to start their own business as The Young Adventurers. They are hired for a job that leads them both to many dangerous situations, meeting allies as well, including an American millionaire in search of his cousin. It started with the sinking of the Lusitania. . . . An American, carrying secret papers, placed them in the care of a young American woman. She was to carry them to England, and advertise for the American: if he did not answer her ad, she should assume he was lost, and place the papers in the hands of the American Ambassador. The American survived, thank God. But the girl whom he entrusted disappeared before she could advertise for him. The papers she carried are lost -- and even now, five years later, the fate of England's postwar recovery depends upon them. . . .