Decades of correspondence between Agatha Christie and her publisher, Billy Collins, have been released and placed in HarperCollins’ newly established archives in Glasgow.

The letters highlight their disagreements over titles and cover designs, despite their long-standing professional relationship.

One letter from February 1946 shows Collins attempting to persuade Christie to accept the cover of The Hollow, writing: “My Dear Agatha, I am so sorry that you do not like the jacket. We all think it is an excellent one and our salesmen are particularly keen on it. You will remember that you did not like Sparkling Cyanide at first, and this turned out to be one of the most popular. Won’t you have second thoughts?”

Christie firmly rejected his plea: “Dear Billy, No. Absolutely no! No reconsideration,” she responded. “And you are wrong about Sparkling Cyanide. I did like the jacket. It was the actual title that I found unsuitable for the book; a flippant title for a really rather serious book. But I do not like a naturalistic jacket.”

The contents of the rejected cover remain unknown, but The Hollow was initially published with a plain red cover before later featuring an illustration of a blonde woman in a bikini holding a pistol over a man.

Despite occasional conflicts, their relationship remained warm. In 1971, Christie wrote to Collins after her 80th birthday celebration: “Dear Billy, I write to send you very many thanks for a lovely and most enjoyable 80th birthday party. It all went off so well and I do want to tell you that I enjoyed myself very much!!! I was delighted to have the photographs; a very good one of you looking most attractive! Your affectionate octogenarian, Agatha.”

Paul Smith, HarperCollins’ archivist, noted the letters reveal both friendship and professional disputes: “Woven throughout the collection is evidence of the beautiful relationship that existed between author and publisher,” he said.

“Despite becoming chairman of the company in 1945, Billy continued to be the primary recipient of her queries, and his affection for her is clear in his responses which frequently open: ‘My Dear Agatha’. Agatha, however, did not allow their friendship to influence issues concerning her work, and she paid Billy few courtesies when she was unhappy with the handling of her novels.”

The archive also contains tributes from political figures. In a 1950 letter from Downing Street, Prime Minister Clement Attlee praised Christie’s craft: “Fifty books! Many of them have 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".

"I admire, also, another of her qualities, one that is not always possessed by those who produce detective stories: her ability, clearly and simply, to write the English language. I am looking forward to the next fifty books.”

Anthony Eden, then Conservative Party deputy leader, also expressed his admiration: “As each of her books is published, it seems that every plot in the field of crime must have been covered. But that is not so. Miss Christie weaves a fresh plot which does not fail to intrigue and interest us all. Long may she continue to enthral us.”