Mr. Splitfoot

From Crimereads: "When it comes to locked-room mysteries, Helen McCloy’s Through a Glass Darkly usually comes in for high praise in critical discussions- and deservedly so. It’s a fabulous entry in her series featuring psychologist Dr. Basil Willing, where the seemingly impossible problem is the appearance of a doppelganger. As such, it has a very sinister, almost dreamlike atmosphere and a haunting ending. However, the McCloy title I’ve chosen to include here is a later work in the Basil Willing series which I feel is more of a locked-room mystery in a literal sense. Like Through a Glass Darkly, though, Mr. Splitfoot does not skimp on atmosphere. The title, for instance, is a reference to none other than Satan himself. This novel is about a haunted room where legend tells it that those who spend the night will be found dead come the morning. As you can imagine, it is a little too easy to find a volunteer willing to put the legend to the test..." - Tom Mead