Correspondence with Billy Collins, going on display this month, reveal a partnership that was extremely lucrative, if sometimes scratchy

Despite her global fame and being feted in the tabloids as “the richest woman writer in the world”, Agatha Christie was no more immune from disputes with her publisher than less successful rivals, according to private correspondence revealed for the first time.

The letters, which are going on show at the Theakston Old Peculier crime writing festival from 20 July as part of an exhibition to mark the bicentenary of Christie’s publisher Harper Collins, reveal that like her famous creation Hercule Poirot, the author was not without vanity.

“I’ve had letters now from different fans expressing surprise that I am ‘such an old lady’,” she complained in October 1949. But it appears what stung the then 59-year-old more than the fan letters was a comment from theatre producer Bertie Meyer, who was staging an adaption of Murder at the Vicarage.

“Frankly, Agatha,” he had announced after seeing her latest publicity image, “that photo makes you look about 70”. Christie replied: “Nobody likes [the photos], possibly, perhaps, because they don’t seem to have been touched up at all? All lines and wrinkles – and dash it all, I’m not 70 – not yet 60.”

The correspondence reveals a close personal bond between the Miss Marple author and her publisher Billy Collins, who had wooed her to the publishing house in 1924 for her breakout novel, The Murder of Roger Ackroyd. It was the start of a long correspondence that lasted until Christie’s death in January 1976. Collins passed away six month later.

Though well into her stride with Poirot by the late 1940s and already hailed as “the Queen of Crime”, her opinion of the covers for her books was not always heeded. Most notably, she complained about a pugnacious-looking Pekingese on the jacket of The Labours of Hercules, which caused much mirth in the Christie household.

In a letter dated 9 April 1947, she complained: “The wrapper design for Hercules has occasioned the most ribald and obscene remarks and suggestions from my family – All I can say is – Try again!!” (sic). Collins was unmoved: the cover reappeared on a recent hardback edition.

Thirty years later, Christie was still wrangling over covers, this time because of a picture adorning Poirot’s Early Cases that looked like he was “going to a funeral and dressed accordingly”, she argued in June 1974.

Facebook Twitter Pinterest Agatha Christie celebrating her 80th birthday in London, 1970. Photograph: The Christie Archive Trust

The correspondence was generally cordial, but on occasion it lapsed into anger. In a 1967 letter, her scrawl reflected “fury” after seeing her latest book in local shops before she had been told its publication date or received copies.

The letter was written the morning after a man at a Detection Club dinner told her he had read the book after buying a copy in Helsinki. “It’s usually [available] in November and then it comes in very handy for sending to friends at Xmas time – but one can hardly send it as that now?” she wrote, adding: “I do think it’s treating your authors disgracefully.”

Such upsets were rare in a bond that proved to be mutually beneficial. By the end of her first decade with Collins, Christie had become his most profitable author and headed a list that defined the inter-war “golden age” of detective fiction.

According to David Brawn, the Christie estate publisher at HarperCollins, Collins’s appreciation of his star performer was so great that he bought her a car in 1953 – after seeing the state of the one she drove. “It was a Humber Super Snipe,” Brawn said. “He had wanted to buy her a Jaguar because it was more fun, but she said she was too old for fun.”

The letters occasionally stray from professional to personal. In September 1940, Collins asked the writer if she could take on his gardener, Midgley, whose wife “has been seedy for some time” and wanted a change in scenery. Christie would later ask Collins, whose brother was a famous tennis player, if he could supply tennis balls through his Wimbledon connections, as they had become increasingly hard to source during the war.

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One aspect missing from the letters is anything about her disappearance in late 1926, which made headlines in the tabloids and is still a subject of speculation. “Those very personal things are not in the correspondence and it was only a short time – a week,” said Brawn.

The exhibition, which will move to Greenways, Christie’s former home in Devon after the crime festival, also features never-before-seen photographs taken on the writer’s archaeological digs, as well as letters from some of her more famous fans, including former Labour prime minister Clement Attlee. On publication of her 50th book, A Murder Is Announced, in 1950, Attlee declared that her work had “beguiled and made agreeable my leisure … I admire and delight in the ingenuity of Agatha Christie’s mind and in her capacity to keep a secret until she is ready to divulge it.”