The Secret Adversary

This is Agatha Christie's second ever mystery book (after "The Mysterious Affair at Styles") first published in 1922. The story begins on the Lusitania, when a young woman - Jane Finn - is given some secret papers before getting aboard a lifeboat. Four years later, young Thomas Beresford and Prudence Cowley meet up in London - both broke and unemployed. Retreating for tea and cakes, they decide upon a plan to become Young Adventurers and take on exciting work. Before long, they are caught up in the mystery of Jane Finn, who has gone missing with the secret papers, putting England in danger.