The mystery of Agatha Christie’s extraordinary appeal is the subject for investigation in this engaging study by Robert Barnard, and by the end of the book you should be a lot clearer about the reasons for applauding the ingenious lady. It’s well-known, of course, that she played around with the original detective-story formula until she’d invented a puzzle of infinite adaptability. Tricks, complications, a great deal of stage management and endless activity all went into it, leaving no room for subtlety in characterisation or distinction in the writing. At best, her literary style is workmanlike; and Barnard argues plausibly that this, along with the clusters of cut-out figures she kept at her disposal, is not the defect it would have been in a different kind of fiction. It’s certain that there is nothing, in Agatha Christie’s novels, to distract you from the intricate workings of the plot.