Compared, not only with Sherlock Holmes and Father Brown, but with Nero Wolfe or Dr Fell or Lord Peter Wimsey, Poirot is a distinctly cardboard character, an obvious artefact. Agatha Christie herself prefers Miss Marple, and her new book, Passenger to Frankfurt, contains neither of them.

The fact remains, however, that Poirot, like a survivor from an almost extinct race of giants, is one of the last of the Great Detectives: and the mention of his name should be enough to remind us how much pleasure Agatha Christie has given millions of people over the past fifty years….

So what is it, this quality which Agatha Christie possesses and so many imitators have lacked?

The secret does lie partly in her plots. The Murder of Roger Ackroyd, Murder on the Orient Express, The ABC Murders and her other classic tours de force deserve their fame. If they seem hackneyed or contrived now or even too easily guessable, that is precisely because they left so permanent an impression on the detective story genre. These books are famous because each of them turns on a piece of misdirection and a solution which, in their day, were startlingly innovatory: but there are many others—Crooked House, Cards on the Table, Death on the Nile, Mrs McGinty's Dead, 4.30 from Paddington—which, in their overall construction, the ingenuity of their clues and the satisfactory smoothness with which their unexpected solutions fall into place, are just as good and perhaps better. It would be silly to pretend that Mrs Christie has never written a bad book. She has—several: but, compared with the size of her output, amazingly few. Almost always, skilled professional that she is, she can out-plot her readers, tripping them up with an extra twist in the tail of the story.

But this isn't all. The real secret of Agatha Christie is subtler. It lies not in the carpentering of her plots, excellent though that is, but in the texture of her writing; a texture smooth and homely as cream. Her books are the easiest of reading. They 'go down a treat', as the saying is.

In a literary sense she doesn't write particularly well. But there is another sense, which for a writer of fiction is perhaps even more important. The ability to buttonhole a reader, to make (as Raymond Chandler put it) 'each page throw the hook for the next', is a separate and by no means common art….

She has one other key quality—the quality of cosiness. There are no nightmares in her books, nothing nasty, nothing horrid, as Jane Austen would say…. This is an important attribute of the true detective story. Its secure and restful formality is part of the pleasure; we don't really have to weep for the victim or for the villain; we ought not to be harrowed, any more than we are by the loss of pieces in a game of chess.

This type of book—and therefore this type of pleasure—has become rare. One reason, the main one probably, is that every new detective story, unlike other kinds of fiction, needs an at least marginally new idea—a new way of committing a murder, or of concealing a murderer's identity, or of solving a murder: and, in the nature of things, finding such new devices gets harder all the time….

Mrs. Christie herself has sometimes ventured a little outside the classic field; Passenger to Frankfurt, as it happens, is an example. But she belongs fairly and squarely in the old tradition….

The Great Detectives were—and, in Mrs. Christie's hands, thank goodness, still are—engaged on a great business. They move, untouched, incorruptible, undefeated, among the mysteries of life and death, teaching us in a parable that there is a reason for everything, that puzzles were made to be solved, that what seems like chaos may be only the observed effects of unknown causes; in short, that the world, instead of being as meaningless as a modern novel, may be like a good detective story, in which the truth and a happy ending are kept for the final chapter.