Crooked House by Agatha Christie

A wealthy Greek businessman is found dead at his London home... The Leonides were one big happy family living in a sprawling, ramshackle mansion. That was until the head of the household, Aristide, was murdered with a fatal barbiturate injection.

Suspicion naturally falls on the old man's young widow, fifty years his junior. But the murderer has reckoned without the tenacity of Charles Hayward, fiance of the late millionare's granddaughter...