In Sparkling Cyanide, Agatha Christie seats six-including a murderer-around a dining table set for seven, one year to the day that a beautiful heiress was poisoned in that very room. Six people sit down to a sumptuous meal at a table laid for seven. In front of the empty place is a sprig of rosemary-"rosemary for remembrance." A strange sentiment considering no one is likely to forget the night, exactly a year ago, that Rosemary Barton died at exactly...