The Circular Staircase by Mary Roberts Rinehart

Rinehart is known as the “American Agatha Christie,” though her first novel predates Christie’s by about 14 years. The Circular Staircase is widely considered the first in the “Had-I-But-Known” school of murder mystery writing, and is also widely considered to be amazing. Plus, she’s the person who originated the phrase “the butler did it.” Can’t argue with that. Library catalog link.