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".