The Secret Adversary by Agatha Christie
The novel that introduced Tommy and Tuppence, a bantering pair of 1920s bright young things who solve a mystery together while falling for each other.
The Great War is over and jobs are scarce. Tommy Beresford and Prudence “Tuppence” Cowley, who were friends before the war, run into each other in London and discover they are both equally short of money and opportunities. On a whim, they decide to start a business, advertising themselves as “The Young Adventurers.” Their first job leads them into a series of increasingly dangerous situations involving international spies, a society beauty, a Russian count, the wreck of the Lusitania, an amnesia patient, an American millionaire, and a fiendishly clever arch-criminal known only as “Mr. Brown.” By the time the dust settles, all the puzzle pieces have been fitted together–and the young couple have realized their feelings for each other and have become engaged.