What was the title of Agatha Christie's first Tommy and Tuppence novel?

The title of Agatha Christie's first Tommy and Tuppence novel was The Secret Adversary.

Published in January 1922, it was Christie's second published mystery novel, following The Mysterious Affair at Styles. The story introduces Thomas Beresford and Prudence “Tuppence” Cowley, two young former wartime volunteers who establish an adventure business after struggling to find work in postwar London.

Their investigation centres on the missing Jane Finn, a secret treaty, political conspirators and the elusive criminal known as Mr Brown. The novel is an adventure thriller as much as a conventional detective story, which helps distinguish it from Christie's later Poirot and Miss Marple mysteries.

A common mix-up is assuming Tommy and Tuppence began in Partners in Crime, because that collection is one of their best-known appearances. In fact, Partners in Crime came later, in 1929, and The Secret Adversary is the series opener.