•Why it earns a slot
Milne's only detective novel is a self-aware, witty entry in the Golden Age genre that openly plays with the Holmes-and-Watson formula while delivering a genuinely surprising solution built on impersonation rather than the standard missing-weapon or alibi puzzle.
When a man is found shot dead in the locked office of an English country house and the owner vanishes, amateur sleuth Antony Gillingham — who happened to arrive moments after the killing — quietly investigates alongside his friend Bill Beverley. The solution turns out to hinge on a secret underground passage, a meticulous impersonation, and a cold act of revenge by the one man everyone trusted.
This distillation is written from the freely available original, which is always the better read when you have the time: gutenberg.org.