The Secret Adversary by Agatha Christie

Full of energy and short of funds, old chums Tommy Beresford and Tuppence Cowley decide to form the "Young Adventurers," and advertise themselves as willing to "go anywhere and do anything." Asked by a British Intelligence official to find a woman named Jane Finn, who disappeared with sensitive government documents, they soon find that their whimsical idea has drawn them into a complicated web of intrigue, intelligence agents, dubious identities, missing government papers, false clues - and all-too-real danger.