In their old age, Tommy and Tuppence Beresford became happy owners of an old country house in the village. Along with the property, they inherited a few odd trinkets, including a collection of old books. And then, one day, as Tuppence was leafing through them, she unexpectedly noticed the underlined words in Stevenson’s novel “The Black Arrow”—and by combining them she could read a phrase: “Mary Jordan died not her own death”…