The Bat & The Circular Staircase. Mary Roberts Rinehart

"The Bat" – Elderly, single Cornelia Van Gorder is renting an old, isolated Long Island mansion owned by the estate of Courtleigh Fleming, a bank president who had reportedly died several months before. On a stormy evening, the electricity flickers on and off. Most of the servants, convinced that the house is haunted, have made excuses and fled. According to a news report, a mysterious criminal known as «the Bat» has eluded police in the area. "The Circular Staircase" – Rachel Innes is a spinster who has had custody of her orphaned niece and nephew since they were children. Halsey and Gertrude talk Rachel into renting a house in the country for the summer. The first night Rachel is there, there is a mysterious trespasser and something falls down the stairs in the middle of the night. The second night, after Halsey and Gertrude have arrived, there is a murder, and Halsey and the friend he has brought to stay disappear. Halsey returns a few days later, without his friend and without an explanation, but by then many other developments have occurred, to the chagrin of the residents.