4.50 from Paddington (Agatha Christie)

When two trains run side by side for just a moment, Elspeth McGillicuddy witnesses something no one should ever see — and then the trains part, leaving her with a memory she can barely prove. No body is recovered. No crime is officially reported. And yet Miss Marple believes her old friend completely, setting in motion one of the most quietly ingenious investigations of her career. The stakes here aren't just about solving a murder; they're about what it means when a woman's account of violence goes disbelieved simply because there's no convenient evidence to support it.

Christie is at her most playful and structurally inventive here, unfolding the mystery largely through a proxy — the resourceful Lucy Eyelesbarrow, hired to infiltrate a household and search for a corpse the police don't believe exists. It's a bold setup that keeps Miss Marple at a remove without diminishing her presence, and Christie uses that distance brilliantly, parceling out clues through letters and observations rather than direct confrontation. The result is a mystery that rewards careful, attentive reading and delivers genuine surprise right through to its final pages.