Mr. Splitfoot

To wake the devil, Lucinda summoned the arch fiend with the ancient invocation, and from the secret room where her friend Vanya had agreed to hide came the eerie response. The rapping called up all the terror of the old tales, and the joke was going marvellously. Until Lucinda realised that Vanya had never arrived at the old house ... 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