The first time Tommy and Tuppence occurred in a Christie novel was in The Secret Adversary (1922). Interestingly, they started out their life as blackmailers (all in search for adventure and money), but the detecting life soon proved just as profitable and exciting.
Their other appearances were in Partners in Crime, a 1929 collection of short stories Christie wrote each reminiscent of another writer's work, N or M a 1940s novel, and Postern of Fate, the last novel Christie ever wrote (as opposed to published).
Unlike many other recurring detective characters, Tommy and Tuppence aged in time with the real world, being in their early twenties in Secret Adversary and in their seventies in Postern of Fate.