The Circular Staircase

Blurb

The Circular Staircase is a mystery novel in the "Had I but known" genre by American author Mary Roberts Rinehart. She wrote the book, which became her first best-seller, at her home at 954 Beech Avenue in Allegheny, Pennsylvania, now part of Pittsburgh.
The "HIBK" genre is associated with several works of Rinehart, specifically The Circular Staircase, in which "a middle-aged spinster is persuaded by her niece and nephew to rent a country house for the summer. The house they choose belonged to a bank defaulter who had hidden stolen securities in the walls. The gentle, peace-loving trio is plunged into a series of crimes solved with the help of the aunt. This novel is credited with being the first in the 'Had-I-But-Known' school."