There isn't much biographical available for Marion Harvey, who was reported to have been born in 1900, but due to early publication information and some performance data, the author appears to be American and the author of several mystery novels and plays between 1922 and 1935. The first of these was "The Mystery of the Hidden Room."
Hidden Room and two other works (one novel, one play) feature Graydon McKelvie, a Sherlock Holmes-worshipping detective. In addition to noting that the criminal device employed in Hidden Room is noteworthy, SS Van Dine once noted (in "The Great Detective Stories," 1927), "The deductive work done by Graydon McKelvie is at times extremely clever."
"The Mystery Of The Hidden Room" is told from the viewpoint of Carlton Davies, whose former fiancee Ruth Darwin was blackmailed into leaving him by the man she ultimately married, powerful banker Phillip Darwin. When the husband is murdered and Carlton finds Ruth standing over him with a gun in her hand one night, she is promptly arrested, tried, and packed off to prison.
Carlton never lost his love for Ruth and is steadfast in believing her innocent of the crime, but the New York City police don't share his convictions. He decides to do his own investigating, but since his butler happens to work with Graydon McKelvie, Carlton begs for McKelvie's help, and the chase is afoot. It doesn't take long for McKelvie to learn that practically no one involved with the case is being honest about their activities on the fatal night and several of the bit players are AWOL.
The hidden room of the title makes its appearance relatively early in the story, thus it's not much of a spoiler for it to be headlined in the title. The room in which Darwin was killed appears to be a locked room scenario with burglar alarms on the windows, but even after the secret room is discovered, there remain many mysteries to solve, including a stoneless ring, a new Will naming a mystery woman as the beneficiary, Darwin's missing nephew, and puzzling sachets sprinkled along the investigative trail. McKelvie also has to solve the mystery of a second bullet that can't be found and a lamp that seems to turn on by itself.
Harvey's writing is de rigueur for her day, with cringe-worthy dialog tags now considered passe and a bit comical ("'Well, I'll be hanged!', I ejaculated"), and hints of racism regarding a black servant and some "chink" goons. But the story runs along at a relatively jaunty clip and, although the eventual culprit isn't a huge surprise if you've been paying attention, the journey to the unveiling is entertaining.