Sparkling Cyanide // Remembered Death — Agatha Christie

It had all started at Rosemary's birthday party. Six guests saw her take a glass of champagne--and die. At first they said suicide. Then they began to remember things: her passionate letter to a mysterious lover, handsome Anthony Browne's threats, and the penniless sister who stood to inherit Rosemary's fortune. Then somebody remembered one thing too many, So the killer struck again!