It was partly thanks to a bet with her sister that Agatha Christie wrote her first detective novel, and 100 years since it was published she is as popular as ever.

Christie was 30 when in February 1920 The Mysterious Affair at Styles was serialised in a British newspaper. The book was published in America in October and the following year in Britain.

Now, according to Guinness World Records, Christie is the world’s best-selling fiction writer and her crime novels have sold 2 billion copies.

“She did that as a woman at the time when women did not really do that kind of thing. And I think that is extraordinary and worthy of... jumping up and down about,” her great grandson James Prichard said.

Prichard said he thought Christie, who died in 1976, didn’t expect to earn a living as a writer and it was just a bit of fun to start with.

That changed when a bet with her sister led to her writing The Mysterious Affair at Styles . “The story came out of a bet with her sister, who bet her that she couldn’t write a book and have it published,” Prichard said.

Famous for characters such as Belgian detective Hercule Poirot and amateur sleuth Miss Marple, Christie’s success is down to the “ingenious plots” that she mapped out in her head, Prichard said.

Christie also wrote 19 plays, of which the most famous, The Mousetrap, opened in 1952. It broke for the first time in March due to the pandemic.

“Our goal is to get all of her written works in all the adapted forms that we can get,” said streaming service Britbox president and CEO Soumya Sriraman.