Four Agatha Christie novels feature Tommy and Tuppence Beresford: The Secret Adversary, N or M?, By the Pricking of My Thumbs, and Postern of Fate.
The pair first appeared in The Secret Adversary (1922), when they were young, unmarried childhood friends drawn into an international espionage mystery. Christie later reunited them in Partners in Crime (1929), but that book is a collection of linked short stories rather than a novel—an important source of counting confusion.
Tommy and Tuppence return as a married couple in N or M? (1941), investigating suspected wartime spies. By the Pricking of My Thumbs (1968) shifts to a darker mystery involving an unsettling elderly resident and a missing woman. Their final appearance is Postern of Fate (1973), in which the now-retired couple investigate a mystery connected with an old book.
The pair therefore appear in five books commonly associated with their adventures, but only four of those are novels. Their full names are Thomas Beresford and Prudence Beresford, née Cowley.