I really enjoyed Plain Murder, as much for the sense of the time in which it was produced and fascination with the attitudes of the author as for the story itself.
This is not really a detective story. We observe all the murderous actions through the eyes of the omniscient narrator: the police, or anyone even vaguely involved in finding out what has happened as part of the developing murderous plotting is barely seen. The narrative pleasure relates entirely to whether the central characters get their comeuppance, and if they do, how this is achieved.
Characterisation is pretty thin: for example, the central character's evolution from thoroughly unpleasant person to serial killing megalomaniac is not subtly charted, but those are not the pleasures this strand of the genre is really expected to give. The story is well-told and engaging, though not really credible in modern terms.
The narrator leaves us in no doubt that everyone in the novel is there to be judged by his God-like superiority: all are his social inferiors, and there is an assumption that his readers share his attitudes and prejudices about class, particularly the 'quaint' absurdities of the aspirations and lives of the central lower class figures. Forester describes Morris et al as 'working class' because it seems to be a term he uses to refer to anyone outside his social ambit: at £5:10s a week and rising, I suspect most of the working class in the 1920s would have regarded Morris as definitely above them.
I find all this fascinating, and a real insight into the social attitudes of Forrester's stratum of the literary classes: the supercilious narrator is almost as sneeringly unpleasant as the young men on whom he focuses. I wouldn't describe this as a 'must-read', but it's interesting, certainly and I could imagine reading it again and other 'mysteries' by Forester, though I doubt I'll ever be tempted to return to Hornblower.