The Silent Pool — Patricia Wentworth

Another case for Miss Silver - sock-knitting sleuth extraordinaire
Adriana Ford, a former actress, has always enjoyed being the focus of attention. Until, that is, that focus seems to be centred on hastening her death. First she felt herself being pushed downstairs. Then there was the bowl of poisoned mushroom soup. When she found one of her tablets tampered with, it was the last straw. She approaches Miss Silver discreetly and is given some sound advice, which she chooses to ignore.
Then Adriana throws one of her celebrated parties and invites everybody she knows. It is only when one of her rather less successful friends is found murdered wearing her coat that Miss Silver is called in to investigate.