Agatha Christie's Secret Notebooks, By John Curran

Agatha Christie created a peculiar world and there are identifiable patterns in her work, as some asides in John Curran's new study of her notebooks reveal: poisoning is the most frequently used method for murder in her books, for example, while doctors are in "statistically the most homicidal profession". Yet Curran shows that this world took patience to construct and could be unpredictable. The plot for Five Little Pigs emerged only after Christie had made 60 pages of notes: "Before that, she had considered a different murder method, a different murderer and different suspects."

The existence of these 73 notebooks wasn't a secret, though they had remained "largely unknown". They contain notes, lists and drafts for many of her novels and plays, interspersed with everyday appointments and travel directions. Curran asserts that there is relatively little order to the notes and they were kept with little eye to posterity: "At her creative peak (roughly 1930 to 1950), her handwriting is almost indecipherable." Indeed, he suggests an element of chaos may have been Christie's secret: "Her strengths lay in her unfettered mental fertility and her lack of system."

She was certainly productive. As Curran notes, "It is possible to read a different Christie title every month for almost seven years." Yet he has carefully produced a book that is readable for those who have not done this experiment. Most chapters are thematic. One deals with novels that use nursery rhymes, for example; another, murders abroad. Selections from the notebooks are given throughout, including games and "exhibits".

Many of Curran's discoveries will shape how Christie is read in future. She worked hard to produce the surprises we remember: "The inspiration for the shock ending [in Endless Night] came to her as she plotted rather than the other way round." And it is wonderful to see how ideas for scenes would come to her almost at random, then be assigned a letter, from A to Z, in the order they would appear.

The book also contains two previously unpublished Poirot stories: "The Incident of the Dog's Ball" and a version of "The Capture of Cerberus". The former is classic Christie, by turns baffling and satisfying. The latter is altogether stranger. Written in 1939, it contains a character who is a barely disguised Hitler. We are not used to a Hitler unknown enough to be a site for imaginative play, making the story a curious and disturbing relic.

This book is the story of a "love affair" between Curran and the notebooks. Its few weaknesses are those of devotion. Though he is brutal in assessing some works, Curran is usually concerned only with whether or not a story is a "first-rate" Christie; there is an absence of wider contexts. Yet this book is fascinating, not least because it demystifies the writing process. In "all the best detective stories", Curran suggests, "the secret of the plot is simple – when you know it". Christie's secret is simple, too, now we know it: this book reveals, above all, how hard she worked.

Tom Sperlinger teaches a course on Agatha Christie's Secrets, which is open to the public, at the University of Bristol