In 1926 Christie was already deeply affected by the death of her mother earlier in the year and the breakdown of her marriage to Archibald Christie. Her brother-in-law, Campbell Christie, suggested that, rather than undergo the strain of composing a completely new novel, Christie should merely compile her most recent series of Poirot stories into a full length book. Campbell helped her revise the stories, which had been written for Sketch magazine, into a more coherent form for book publication. His assistance mainly took the form of revising the beginnings and ends of the stories to make them flow better into a novel - the substance of each story remains the same between the short story version and the novel version. Unlike the later Partners in Crime (1929), the order of the stories was retained.
Around this time, a novel was offered for publication to The Bodley Head and was rejected. No other information exists on this novel, but Zemboy suggests it was The Big Four. His theory continues with Christie's relationship to her new publisher William Collins, Sons. Christie would have realized that The Big Four was an inferior novel and went to work writing The Murder of Roger Ackroyd for the new publisher.
The book was published a few weeks after the disappearance and reappearance of Christie. The resulting publicity over her name caused the new novel to become a sales hit. Sales were good enough to more than double the success of The Murder of Roger Ackroyd. It sold despite not being a traditional murder mystery, being a tale of international intrigue and espionage, and opening the possibility of more spy fiction from Christie.
In 1942, Christie wrote to her agent, Edmund Cork of Hughes Massie, asking him to keep a manuscript in reserve (probably Sleeping Murder) and statedI have been, once, in a position where I wanted to write just for the sake of money coming in and when I felt I couldn't - it is a nerve wracking feeling. If I had had one MS 'up my sleeve' it would have made a big difference. That was the time I had to produce that rotten book The Big Four and had to force myself in The Mystery of the Blue Train.