The Yellow Room Conspiracy — Peter Dickinson

Wild, handsome Gerry Grantworth died mysteriously in the locked, gas-filled Yellow Room at Blatchards, the famous Vereker mansion. He had been the greatest passion of Lucy, one of the five beautiful Vereker sisters, and the best friend of her lover, Paul. Gerry had given both Lucy and Paul reason to kill him, and thirty-six years later, Lucy is dying and asking her lover for one last confession. But as the couple compare accounts of that fatal day, along with the mad years of World War II, sexual liaisons, political scandals, and intimate secrets that preceded and followed it, they piece together a deadly puzzle. Only now does a chilling truth emerge: the long-hidden solution to a crime born on the dark side of upper-class British life.