They appear together in four full-length novels and one collection of short stories. Their other appearances were in Partners in Crime, a 1929 collection of short stories (each reminiscent of another writer's work); N or M? , a 1941 espionage novel; By the Pricking of My Thumbs (published in 1968); and Postern of Fate in 1973, the last novel Christie ever wrote (although not the last to be published).
Tuppence appears as a charismatic, impulsive and intuitive person, while Tommy is less imaginative, and less likely to be diverted from the truth (as their first adversary sums him up "he is not clever, but it is hard to blind his eyes to the facts"). They therefore make a good team. It is in this first bookThe Secret Adversary that they meet up after the war, and come to realise that, although they have been friends for most of their lives, they have now fallen in love with each other.
Unlike many other recurring detective characters, including the better known Christie detectives, Tommy and Tuppence aged in time with the real world, being in their early twenties in The Secret Adversary and in their seventies in Postern of Fate. In their early appearances, they are portrayed as typical upper middle class "bright young things" of the 1920s, and the stories and settings have a more pronounced period-specific flavour than the stories featuring the better known Christie characters. As they age, they're revealed to have raised three children -- twins Deborah and Derek and an adopted daughter, Betty. Throughout the series they employ a man named Albert, who first appears as a lift boy who helps them in The Secret Adversary, and in Partners in Crime becomes their hapless assistant at a private detective agency; by Postern of Fate he's their butler and has been married and widowed. In Postern of Fate they also have a small dog named Hannibal.