8. Postern of Fate, 1973 

Postern of Fate brings back the beloved detective duo Tommy and Tuppence Beresford in their twilight years as they uncover a mystery linked to their new home’s previous occupants. 

The story is sentimental and retrospective, as the couple revisit clues from their youth, making it more of a nostalgic character study than a suspenseful whodunit. 

Longtime readers might sense an air of finality in this book, which is often seen as Christie’s farewell, both to her characters and to the world of mystery fiction itself.

6. Passenger to Frankfurt, 1970

A political thriller featuring diplomat Sir Stafford Nye, who becomes embroiled in an international conspiracy while helping a mysterious woman at an airport. 

Influenced by the political climate of the late 1960s, Passenger to Frankfurt addresses unlikely themes of youth rebellion, fascism, and power-driven conspiracies in post-war Europe. 

5. Third Girl, 1966

In Third Girl, Poirot steps into the distinctly modern world of drug, art and fame, when a young Londoner consults him, but visibly doubts his capabilities because of his age. 

The mystery centres on the young woman's claim that she 'might have' committed murder, a shift from Christie’s typical whodunit formula to something closer to a psychological thriller.

4. The Clocks, 1963

Here, Poirot takes on an unusual investigation involving a typist who finds a dead man in a client’s house surrounded by an assortment of clocks, none of which tell the correct time.

However, the celebrated detective remains in the background for much of the story as local detective Colin Lamb takes the spotlight, while Christie’s own interest in Cold War themes is reflected in the espionage-heavy narrative.