Black Widow. S. Fowler Wright

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?