Of course nobody is expected to care in any humanist sense: it is, quite simply, that one has to know. Agatha Christie at her best writes animated algebra. She dares us to solve a basic equation buried beneath a proliferation of irrelevancies. By the last page, everything should have been eliminated except for the motive and identity of the murderer; the elaborate working-out, apparently too complicated to grasp, is suddenly reduced to satisfactory simplicity. The effect is one of comfortable catharsis.

During the Second World War, just after finishing The Body in the Library, Agatha Christie wrote two novels which she intended to reserve for publication after her death. They described the last cases of her two most famous detectives, Poirot and Miss Marple. Now she has generously decided to release at least one of them while she is still alive. Curtain, therefore, belongs to the period when...