The Big Question: How big is the Agatha Christie industry, and what explains her enduring appeal?

Why are we asking this now?

Because Greenway, the sprawling Devon mansion where Agatha Christie used to enthral her guests with night-time readings of her unfinished manuscripts, has been opened to the public for the first time. Known locally as Mrs Mallowan, Christie used the 121-hectare coastal estate as a holiday home.

The cream-coloured Georgian mansion has become to her fans what 221b Baker Street is to Sherlock Holmes enthusiasts. The National Trust spent £5.4m restoring the house, which was the setting for the Hercule Poirot mystery Dead Man's Folly and was where Christie spent every summer from 1938 to her death in 1976 during her second marriage to the archaeologist Max Mallowan. At least three of her novels are set there, but Christie tended not to write her books at Greenway, which she regarded as a place of recreation where she could take time off from being the Queen of Crime.

Why has it only just opened?

Until relatively recently Greenway remained in the hands of Christie's daughter Rosalind and her second husband Anthony Hicks. After their deaths in 2004 and 2005, Matthew Pritchard, Christie's grandson, donated the house and all its contents to the National Trust but, as the house itself was crumbling away, it needed hefty restoration work before it could be opened. It will likely be a major draw for the millions of Christie fans worldwide and, come the summer, the National Trust expects at least 600 people to visit the house very day.

So just how popular are Christie's books?

Well, the Guinness Book of Records says that, alongside Shakespeare, Christie is the best-selling author of all time. If you disregard holy texts such as the Bible and the Koran there are probably more books in the world today by Christie than by any other author. In all she is thought to have sold between two and four billion books worldwide, and every year a further five million novels are bought.

Who is now buying Christie's books?

People all over the world. Part of the reason Christie sells so well is that her works have been translated into virtually every major language, giving all nations unprecedented access to the 127 books and 15 plays she churned out between 1920 and 1976. She is so comprehensively translated that Unesco last year named her the most translated author in the world.

How successful were her plays?

Again, Christie is something of a record-breaker when it comes to her stage work. The Mousetrap, a murder mystery play which was inspired by the real-life death of a boy in foster care, has had the longest initial run of any play in the world. It opened in 1952 at the New Ambassadors Theatre and when that closed in 1974, it immediately transferred to St Martin's Theatre next door, thereby keeping its initial run status. A wooden counting board in the theatre's foyer is used to keep track of every performance. Yesterday's 3pm matinee was the 23,437th performance. During this run there have been no fewer than 382 actors and actresses appearing in the play, 116 miles of shirts have been ironed and more than 415 tons of ice cream sold.

What is it about Christie that makes her work so popular?

According to Barry Forshaw, the editor of British Crime Writing; An Encyclopedia, it is the simple language, meticulously constructed plots and the evocative settings of a bygone British era that keep people coming back. "The first thing that makes Christie so accessible is the way she writes," he says. "There is no single author out there who manages to translate so well into so many different languages. She keeps the language fairly straightforward and simple but the plots are constructed incredibly well, like a finely tuned machine. Then there are the settings. She writes about an England that people nurture in their minds even if it never really existed and that is something that appeals to both British readers and worldwide fans."

What did Christie think about her own work?

Considering she was dubbed the "Queen of Crime" and achieved worldwide fame at an early stage in her writing career, Christie spoke of her own work in remarkably self-depreciating terms. Her detractors have often criticised her for lacking substance – P D James once famously blasted her for her "cardboard cut-out" characters – but Christie described herself as an entertainer, not a literary giant. She was also unaffectionate towards one of her most famous creations, the Belgian detective Hercule Poirot who, by the 1960s, she felt had become "insufferable" and "an egocentric creep".

How was she able to write so many bestsellers?

When asked what her method was for churning out such readable books at such a rate Christie once replied: "The disappointing truth is I haven't much method." She thought that three months was enough time to write a book and suggested that plays should be written much more quickly. "I think the real work is done in thinking out the development of your story and worrying about it until it comes right," she once said. "That may take quite a while. Then, when you've got all your material together, all that remains is to find time to write the thing."

Who now makes the money from her sales?

Exact figures are difficult to come by but royalties from book sales alone are thought to be worth at least £5m a year. The company that benefits the most is Chorion Ltd, which paid £10m for a controlling share of the rights to Christie's work back in 1998. Chorion already owned the rights to several big-name children's titles including Enid Blyton's Noddy and Famous Five series, and Roger Hargreaves's Mr Men books. The company's relaunch of Christie's novels in 2002 was so successful that the author's most famous work, And Then There Were None, appeared once again on the US bestseller list and sold out its initial print run in just 10 weeks.

What about television rights?

Films and television series based on Christie's works have played just as pivotal a role as her books in introducing successive generations to her gripping whodunits. The portrayals of Poirot by Peter Ustinov and David Suchet, for instance, have become just as recognisable as the books themselves and it seems that our appetite for televised Christie is as undiminished as ever.

Are there any new films in production?

Most certainly. Last year Chorion agreed a deal with ITV to make eight Christie films, including an adaptation of Murder on the Orient Express. The contract calls for four Miss Marple films, starring Julia McKenzie, and four Poirot films, with David Suchet. The films are expected to be broadcast in the next 18 months.

Is Agatha Christie anything more than a skilful purveyor of pot-boilers?

Yes...

* Her books are masterfully crafted whodunits that deserve respect

* She created a blueprint for crime fiction that set a benchmark for all future writers of the genre

* The sheer global popularity of her works show how enduring and overwhelming her appeal is

No...

* Popularity is not always an indicator of quality

* She has been routinely criticised by leading writers for having an "undistinguished style" and one-dimensional characters

* Her works simply cannot compete with her more cerebral literary peers