Behold, here's poison - Georgette Heyer

When Gregory Matthews, patriarch of the Poplars is found dead one morning, imperious Aunt Harriet blames it on the roast duck he ate for supper. After all, she had warned him about his blood pressure. But a post-mortem determines that the cause of death is much more sinister. Murder. By poison.

Suspicion falls immediately amongst his bitter, quarrelsome family. Each has a motive; each, opportunity. It falls to Superintendent Hannasyde to sift through all the secrets and lies and discover just who killed Gregory Matthews, before the killer strikes again...