The Moonstone Castle Mystery — Carolyn Keene

When Nancy Drew receives a valuable moonstone as a gift from an unknown person, she is amazed and puzzled. But it is only the first of several startling events in this complex mystery that challenge the ingenuity of the pretty sleuth.

Why are the Bowens -- a missionary couple who recently returned to the United States -- having so much trouble finding their missing seventeen-year-old granddaughter? Trying to uncover a clue to the orphan girl's whereabouts, Nancy and her friends travel to Deep River, the town where young Joanie Horton lived with her guardian grandmother until Mrs. Horton's death fourteen years ago.

From the motel where Nancy, Bess, and George stay, they see in the distance an intriguing castlelike structure with a drawbridge. Gossipy Mrs. Hemstead at the village tearoom insists that Moonstone Castle is haunted. Curious, the three girls attempt to explore the abandoned castle, but an ominous voice warns them away.

Other strange happenings in Deep River convince Nancy that there is a connection between Moonstone Castle and the mysterious moonstone gift. But what is the significance? And where does the baffling disappearance of Joanie Horton fit into the intricate puzzle?

How the young detective exposes a cruel hoax that has far-reaching results makes a dramatic and exciting story.