Agatha Christie 
Secret Adversary 

The book introduces the characters of Tommy and Tuppence who feature in three other Christie novels and one collection of short stories; the five Tommy and Tuppence books span Agatha Christie’s writing career. The Great War is over, and jobs are scarce. Childhood friends 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. (Wikipedia)