The Case of the Disappearing Diamonds — Carolyn Keene

WHEN A FRIEND IS SET UP TO TAKE A FALL, NANCY MUST DEAL IN DIAMONDS AND DANGER

Famous mystery author Monica Crown is mixed up in a real-life case more puzzling and frightening than any she's ever devised. Her daughter Karen has been accused of stealing a fabulous set of six diamonds. When Mrs. Crown asks Nancy to help prove her daughter's innocence, the author's car takes a mysterious plunge into a canyon -- and her body vanishes into thin air!

Nancy sets out to find the missing diamonds and the missing mystery writer, and finds plenty of trouble along the way. A sinister white automobile, a rampaging motorboat, and a sabotaged airplane lead to a fiery climax in the strange case of the disappearing diamonds.