She was the "Queen of Crime", whose murder mysteries centred on a final "big reveal". But newly seen correspondence between Agatha Christie and those who staged her plays, including her most famous, The Mousetrap, show that, despite her stature as a master and innovator of the crime genre, theatre producers were much less in awe of her star status.

Correspondence between Christie and the theatre director and producer behind the stagings show that they believed her endings could be slow and laborious.

One letter, sent in 1961 by Christie's producer, Peter Saunders, to her director, Hubert Gregg, discussing Christie's little-known play The Patient, criticised the final scenes for being too drawn out – and even went so far as to mock the playwright when he suggested that the denouement might be changed. "Donald Walker feels the present ending is too long and wordy and also that it needs breaking up," the letter says. "I also send, for your own amusement, his own suggestion of how he thinks the play ought to finish. Can you imagine Agatha's face?"

Download the new Independent Premium app Sharing the full story, not just the headlines

The Agatha Christie archive, which includes annotated typescripts, staging notes and personal letters, goes on sale at the London International Antiquarian Bookfair later this month.

A letter to Gregg from the author, whose books are estimated to have sold over four billion copies around the world to date, describes the "disheartening" experience of her 1960s flop, Go Back for Murder, revealing her sensitive and proprietorial feelings towards the play's production: "If their girl [the actress] could act – but when they can't, they can't," she wrote, adding that "it wouldn't have made a difference to the critics anyway!".

Christie likened her own plays to "race horses" which are "always either amazing or disappointing their stables!".

The Hollow The Unexpected Guest Rule of Three The Mousetrap The Patient

Alongside the success of The Mousetrap, Christie continued to write fiction, publishing one or two books a year until her death, at 85. She is described in Guinness World Records as the bestselling novelist of all time, publishing a total of 66 murder-mystery novels in her lifetime. The first, The Mysterious Affair at Styles, introduced the Belgian detective Hercule Poirot.

James Hallgate, of Lucius Books in York which is selling the archive, said Christie's "obvious sensitivity to critics" made the letters particularly unique.

He said: "She refers to Gregg as 'a real friend' and feels able to discuss the successes and failures of her stage adaptations with him in these letters, thus offering an intimate vantage point on the vagaries of taking a script from page to stage and in the process handing it over – or not – to the director and actors."

The Agatha Christie Archive is on sale at Lucius Books for £16,000. The bookseller will be exhibiting the collection at the London International Antiquarian Bookfair, at Olympia, 22-24 May