A literary critic was once relaxing on an Italian beach when he saw a middle-aged woman reading a Miss Marple thriller in translation.
Two days later, he was north of the Arctic Circle and spotted a teenage girl in a deck chair, with the same book, this time in Norwegian. Here, he concluded, was evidence that no British writer has ever enjoyed such universal appeal as Agatha Christie.
Her first novel - The Mysterious Affair at Styles, which introduced us to a Belgian detective called Poirot - was published in 1920, and by the time she turned 60, she had shifted 50 million books worldwide.
Even after her death in 1976, her name continued to sell books in almost every country on Earth. She was, wrote a fellow author, the "best of all sellers - abstract, enigmatic, logical and completely ruthless".
Yet among people who fancy themselves as great intellectuals, Christie's popularity has always been a standing affront.
She was very conscious of her reputation: when awarded the CBE in 1956, she told a friend: "I feel it's one up to the Low Brows!"
It is hard to think of any author who has been so successful for so long while inspiring such contempt. In 1944, US critic Edmund Wilson claimed that her prose was "of a mawkishness and banality that seems to be almost literally impossible to read".
For the Guardian's high priestess, Polly Toynbee, Christie's books are "suffused with a peculiar English snobbery" and "firmly set among the middle classes, on the uncomfortable presumption, perhaps, that the lower classes are too boring to write books about or that crime among them is too common to merit attention".
Even her fellow crime writers, no doubt envious of her sales, love to sneer.
Ruth Rendell said that whenever she read an Agatha Christie: "I don't feel as though I have a piece of fiction worthy of the name in front of me", while P.D. James described her simply as "such a bad writer".
But as anyone who has watched the BBC's brilliant adaptation of Christie's best-selling book, And Then There Were None, will know, the charge that she is a bad writer, or even a snobbish one, is wrong.
It is simply not true that her books are extended apologias for the upper classes. Her aristocrats are usually faintly ridiculous, and some of her conservative business tycoons turn out to be murderers.
In And Then There Were None, some characters are snobs. But their prejudice is met with the ultimate penalty: a memorably gory death.
And who, reading the novel on which the riveting TV version is based, could possibly think Christie is a bad writer?
The characters are briskly and expertly drawn, from the anxious, guilt-ridden Vera Claythorne to the ruthless, dissolute Philip Lombard - superbly played by Aidan Turner.
The momentum builds with chilling intensity. The atmosphere is tense, glowering, heavy with the ghosts of murders past and future. And the solution is frustratingly simple and ingeniously clever.
Oddly enough, Christie's best-selling whodunnit does not feature a detective, which makes it all the more unsettling. Yet Christie's detectives are magnificently memorable.
The most famous, of course, is Poirot, a strange figure compared with his crime-solving predecessors. Most heroes of detective fiction in the 20s and 30s tended to be foppish Oxbridge types. Ngaio Marsh's protagonist was the gentleman detective Roderick Alleyn, while Dorothy L. Sayers created the immensely irritating Lord Peter Wimsey (Eton and Balliol).
Christie, by contrast, established her reputation with a Belgian immigrant only 5ft 4in tall (1.62m), who lives in a modernist flat rather than a stately home and is physically incapable of even the slightest heroics.
The fact she created such an unconventional hero ought to make us question the ill-informed stereotype that her books are about people with cut-glass accents sipping cocktails on country house lawns.
Unlike some of her competitors' stories, her books were written for the very kind of people who might read them in the lounge, not the drawing room. The typical setting is a small provincial town.
It is true her characters are overwhelmingly middle class - secretaries, salesmen and shop girls as well as retired colonels and vicar's ladies. But she rarely treats them condescendingly or sneers at them for being suburban.
Domestic servants are treated sympathetically, and are almost never the killers. The charge of snobbery simply does not stack up.
So let's move on to another serious accusation - that Christie is just a one-dimensional entertainer. She was, in fact, an extremely clever writer, and brilliant at plots and setting puzzles, as even her critics have to concede.
But what about her wooden characters, the naysayers persist, those dreadful stereotypes? This is to miss the whole point about Christie.
Time after time, she feeds us stereotypes only to trick us. She presents her characters as stock figures before, like a true artist, revealing the complexity beneath the facade.
The final insult is that Christie was ignorant of real life. Certainly, she grew up in upper-middle-class comfort, born in Torquay in 1890 to a well-off American stockbroker and his Belfast-born wife.
Yet her father was dogged by illness and died when Agatha was just 11. Her biographers agree her detective novels reflect a longing to banish the emotional instability that blighted her early life.
This childhood tragedy also explains the chaos that so often erupts inside her fictional families, with violent death shattering the illusion of domestic respectability.
What critics also forget is that the prim young lady was a volunteer nurse during World War I. She knew at first hand what violence meant.
Even the love triangle - her most effective narrative device - was firmly rooted in reality, specifically, the trauma of her divorce from her first husband, the debonair Archie Christie, who in 1926 abandoned her for a much younger woman.
It was this betrayal that precipitated her vanishing act, when she disappeared from her Surrey home, leaving her car by the edge of a lake, and took refuge in a hotel in Harrogate.
And though she would marry the archaeologist Max Mallowan, her obsession with infidelity suggests that the scars never truly healed.
One common misconception is that her books represent a kind of escapism. Yet what is most striking is the exact opposite - the ordinariness of her characters and their settings, from a rural vicarage to a suburban villa. As Christie knew, most murders are committed within the family.
And, for all the complexity of her puzzles, her murderers' motives often boil down to those two timeless - and classless - favourites: sexual jealousy and greed. Even her murder weapons are strikingly mundane: poison, a steak knife, even a kitchen skewer. Christie loved taking everyday items and turning them into instruments of death. What her books offer is the violence of the 20th century, toned down and introduced into the heart of the suburban household.
On screen, producers have tended to favour Christie books set in exotic locations, such as Murder On the Orient Express and Death On the Nile. For my money, her most effective stories tend to have more humdrum settings - the fictional villages of St Mary Mead, Chipping Cleghorn, Much Deeping, Wychwood-under-Ashe.
On the surface, everything seems civilised, quiet and terribly English. And yet, especially in her books written after World War II, this is a world being transformed by social change.
Under the pressures of war, austerity and socialist reform, the threads that bound English life were unravelling, leading to an increasingly atomised, anonymous society, in which no one could be entirely sure of anyone else.
Enter Miss Marple - the very picture of a little old lady, with "snow white hair, a pink crinkled face and very soft, innocent blue eyes".
It is a shame the screen adaptations of the Miss Marple books, from the galloping Margaret Rutherford films to the terrible ITV series starring Geraldine McEwan and Julia McKenzie, have encouraged a misleading image of Christie's most compelling detective. There is nothing cosy about Miss Marple.
"I'm afraid I have a tendency always to believe the worst," she says in A Murder Is Announced.
The moment of death is rarely shown. Christie is interested in motive, not murder. Her killers are not freaks or lunatics. In her fictional universe, what makes evil so frightening is that it appears so normal.
As And Then There Were None shows, we are all capable of killing. And this is the point of her detective novels. No one, not the policeman, the detective or even the narrator, can be entirely above suspicion.
"There are such strange things buried down in the unconscious," says a character in Appointment With Death. "A lust for power, a lust for cruelty, a savage desire to tear and rend. We shut the door on them and deny them conscious life, but sometimes they are too strong."
When another character describes someone as "quite harmless", the detective Poirot says gently: "Is anybody - ever - quite harmless?"
This is what the TV dramatisation of And Then There Were None has revealed with such ruthless brilliance. The terrible, chilling truth - not just about the murders at Soldier Island, but about human nature itself.
One of the most famous fictional characters of all time, the inimitable Belgian private detective is synonymous with waxed moustaches, perfectionism and little grey cells. Poirot would be the first to call himself a great man - he has never been known for his modesty - but with such success in his career, it is difficult to argue with him.
Miss Jane Marple doesn't look like your average detective. Quite frankly, she doesn't look like a detective at all. But looks can be deceiving ... For a woman who has spent her life in the village of St Mary Mead, Miss Marple is surprisingly worldly. But, as she often points out, she has had every opportunity to observe human nature.
Tommy and Tuppence
International spies, two world wars, murders, thefts and not to mention marriage, Tommy and Tuppence seek out excitement wherever it may lie. Tuppence leads the way with her charismatic nature, while Tommy's slow, considered manner provides the perfect foil. They form the "Young Adventurers Ltd" and the adventures begin.