Hercule Poirot's Christmas (1939)

The violent death of a domineering father leaves Poirot with a disturbed room, family resentments and an apparently sealed crime scene. Christie integrates the physical puzzle with what people believe about one another, giving the eventual explanation more to resolve than a troublesome door. A good bridge from ordinary family whodunnits into locked-room fiction. Expect a cruel household and a bloody murder rather than a comforting Christmas gathering.