Wicked Uncle (Miss Silver, #12)

Also published as Spotlight.

When the menacing Gregory Porlock invites a group of unacquainted friends to his home, the house party takes on a sinister edge. The host himself is stabbed in the back, and it becomes clear to all that he was a blackmailer. But which of his unfortunate comrades would have thrust the fatal knife is up to Miss Silver to discover. Only she can see through the lies and motives, the clues and questions, to determine which guest made certain the party was over.