The A.B.C. Murders (1936)

An alphabetical series of killings invites the reader to look for a pattern alongside Poirot. Christie makes that invitation a reason to examine the assumptions built into an investigation, not simply to predict the next letter. Its wider geography and changes of viewpoint give it more momentum than a static country-house case. Track individual circumstances as carefully as the sequence, without assuming that a conspicuous organising principle explains everything.