The Secret Of Annexe 3 by Colin Dexter

During a New Year’s Eve fancy-dress party at an Oxford hotel, a guest is found dead in a separate annexe, and the case turns on swapped rooms, misleading costumes, and shaky alibis. A sardonic inspector and his patient sergeant sift through red herrings, cryptic clues, and tangled relationships to expose a carefully plotted murder rooted in old grievances—and the hidden truth that the annexe was meant to conceal.

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