Fair warning: There are two things you must do if you know nothing of the book: discuss it with no one, and read it with all speed.

• H.R.F. Keating: ‘One of the landmarks of detective literature’ (in his Crime & Mystery: The 100 Best Books).
• Julian Symons: ‘The most brilliant of deceptions’ (in his Bloody Murder: From the Detective Story to the Crime Novel).
• Irish Independent: ‘A classic—the book has worthily earned its fame.’

5. The Big Four (1927)

Hercule Poirot is preparing for a voyage to South America. Looming in the doorway of his bedroom is an uninvited guest, coated from head to foot in dust and mud. The man’s gaunt face registers Poirot for a moment, and then he collapses. The stranger recovers long enough to identify Poirot by name and madly and repeatedly scribble the figure ‘4’ on a piece of paper. Poirot cancels his trip. An investigation is in order. Fortunately, Poirot has the faithful Captain Hastings at his side as he plunges into a conspiracy of international scope—one that would consolidate power in the deadly cabal known as ‘The Big Four.’

6. The Mystery of the Blue Train (1928)

Le Train Bleu is an elegant, leisurely means of travel, and one certainly free of intrigue. Hercule Poirot is aboard, bound for the Riviera. And so is Ruth Kettering, the American heiress. Bailing out of a doomed marriage, she is en route to reconcile with her former lover. But her private affairs are made quite public when she is found murdered in her luxury compartment—bludgeoned almost beyond recognition. Fans of the later novel Murder on the Orient Express will not want to miss this journey by rail—and Poirot’s eerie reenactment of the crime…

7. Black Coffee (1930; 1998)

Sir Claud Amory’s formula for a powerful new explosive has been stolen, presumably by a member of his large household. Sir Claud assembles his suspects in the library and locks the door, instructing them that the when the lights go out, the formula must be replaced on the table—and no questions will be asked. But when the lights come on, Sir Claud is dead. Now Hercule Poirot, assisted by Captain Hastings and Inspector Japp, must unravel a tangle of family feuds, old flames, and suspicious foreigners to find the killer and prevent a global catastrophe.

Of note: Black Coffee was Agatha Christie’s first playscript, written in 1929. It premiered in 1930 at the Embassy Theatre in Swiss Cottage, London, before transferring the following year to St Martin’s in the West End—a theatre made famous by virtue of its becoming the permanent home of the longest-running play in history, Agatha Christie’s The Mousetrap. Agatha Christie’s biographer, Charles Osborne, who, as a young actor in 1956 had played Dr Carelli in a Tunbridge Wells production of Black Coffee, adapted the play as this novel in 1998.

• Antonia Fraser, Sunday Telegraph: ‘A lively and light-hearted read which will give pleasure to all those who have long wished that there was just one more Christie to devour.’
• Mathew Prichard, from his Foreword to Black Coffee: ‘This Hercule Poirot murder mystery…reads like authentic, vintage Christie. I feel sure Agatha would be proud to have written it.’