The Circular Staircase (1908) by Mary Roberts Rinehart is a classic "had-I-but-known" mystery centered on Rachel Innes, a strong-willed spinster who rents a country mansion for the summer with her niece and nephew. Almost immediately, strange noises, midnight intruders, and unsettling events plague the house. When a murder occurs on the grounds, Rachel is pulled into a web of secrets involving hidden passageways, financial crimes, and long-buried scandals. As she investigates, the truth behind the eerie happenings-and the significance of the circular staircase-slowly unravels, revealing a conspiracy that connects everyone in the house.