The Clue in the Crossword Cipher

Book forty-four in the Nancy Drew Mystery Stories series, first published in 1967. It sends Nancy to Peru to decode a cipher carved into an ancient wooden plaque, before a smuggling ring can reach the treasure it points to.

Carla Ponce, a young woman living in Peru, invites Nancy, Bess, and George to help solve her family's "monkey mystery." It centers on a wooden plaque bearing a crossword cipher that time has largely worn away. A gang led by a smuggler known as El Gato steals the plaque, so Nancy has to recover it before she can even begin decoding it. Her investigation carries her to the Incan ruins at Cuzco and Machu Picchu. There she and her friends help break up the smuggling ring and turn up an archaeological find of their own.

Carolyn Keene is the house pseudonym the Stratemeyer Syndicate, and later Grosset & Dunlap and Simon & Schuster, used across the entire Nancy Drew series. Individual volumes were written by a rotating set of ghostwriters under contract, beginning with Mildred Wirt Benson on the earliest titles. This catalogue follows its own convention for house pseudonyms and credits the byline as printed, not an individual ghostwriter.