The Circular Staircase

When Rachel takes a country house for the summer with her niece and nephew, she expects to fill her days with relaxing social activities. But these plans take a turn for the sinister when, on the second night, a dead body is found at the bottom of the house's circular staircase.
Mary Roberts Rinehart's The Circular Staircase is credited with creating the "Had I But Known" genre of detective fiction.