NY: Brentano’s, 1918.
Magnay, a baronet, was a popular and prolific novelist from the 1880s through World War I but almost no one has heard of him today. His prose couldn’t be called scintillating, nor his plots ingenious, but both were workmanlike and his books were very readable. This classic locked-room mystery was (I think) his last and was published posthumously the year after his death. And while it’s not a bad story, it’s probably best approached as an artifact of early 20th-century social history.