LONDON: If there lurks anywhere in an Agatha Christie novel a small boy, perhaps one who enjoys tennis or cricket and loves spending his school holidays at his granny’s large house on the Devon coast, then that boy is almost certainly Mathew Prichard, the only child of the only child of the prolific author known as the queen of crime. Oddly, Mathew says he has never been asked before whether he has a cameo role in any of his grandmother’s 80 books, but he rather thinks not. “It’s not the kind of thing she would have done,” he says. “My grandmother was fiercely private, especially when it came to her family.”

But despite, or perhaps because of, her famed reclusiveness, the writer’s private life has been the subject of endless fascination since her death in 1976. Now Mathew, 68, has provided a rich new seam of material with a book that describes a previously uncharted chapter of her life a round-the-world trip she made in 1922. The book is in Christie’s own words: it’s based on letters she wrote to her mother, Clara, as she traversed the globe with her husband, Archie, for 10 months, having left their two-year-old daughter, Rosalind — Mathew’s mother at home in England.

The letters, Mathew explains, lay undisturbed in boxes at Greenway, Agatha’s house in Devon, for many years until his mother died in 2004; but he always knew he would use the material for a book. The letters provide almost the only clues to one of the most baffling episodes in Agatha’s own life her doomed marriage to Archie Christie, who left her for another woman within a few years of their return to England. Mathew had a fascinating reason for wanting to know more about that because he never met his grandfather, although Archie didn’t die until the 60s. When Mathew was a baby, his father — Rosalind’s husband was killed in the Normandy landings, so a relationship with his maternal grandfather could have been important to both of them. Why didn’t it happen?

“I don’t think my grandmother would allow it,” says Mathew. “She wasn’t even keen on Rosalind seeing Archie, though she could hardly prevent that he was her father, after all. But she really didn’t want me to be close to him.”

Are we talking revenge? Could it be that Agatha who built an incredibly successful career on the study of how determined and devious people can be when it comes to getting their own back found a way of wounding Archie, decades after she was abandoned so cruelly? Her much investigated “disappearance” for 11 days in 1926, after Archie told her he was leaving, roused speculation that she was trying to make it look as if he had killed her. “I don’t much like the word revenge,” says Mathew. “But yes, I think she was possibly getting her own back. Archie had hurt her, and I don’t think she ever forgot that. And it’s certainly very unusual for a child never to meet his grandfather.”

What emerges from the book is how very different Agatha and Archie were. His letters are short, fairly formal and to the point: her letters are long, effusive, and full of colourful descriptions of where they were and what they were doing surfing was one enthusiasm and she was one of the first Britons to surf standing up.

They went to South Africa, Australasia, Honolulu and Canada, and Agatha basked in a new-found love of travel. The letters also show her starting to hone her skill of building characters: the Christies made their trip as members of a trade mission to promote the British Empire Exhibition, held in London in 1925 the leader of the expedition, a Major Belcher, turned out to be, in Agatha’s words to her mother, “a most unpleasant man” with an odious temper and delusions of grandeur who managed to upset the local dignitaries at every port. “You can’t imagine how farcical it all is!” writes Agatha.

By arrangement with Guardian