Black Widow by S. Fowler Wright (epub/Kindle/pdf)
When Inspector Pinkey investigates the murder of Sir Daniel Denton in his country home, he's confronted with three prime suspects: the man's widow, his brother, and a recently-fired servant. But everywhere the policeman looks, he finds conflicting evidence--and the individuals involved all hate each other. Then Gerard Denton apparently commits suicide--or is it murder?--and the servant suddenly disappears. Is Lady Denton the murderer by default?
An absolutely stunning classic crime novel, originally published as Who Else But She?