The Thirteenth Pearl is the fifty-sixth volume in the Nancy Drew Mystery Stories series. It was first published in 1979 under the pseudonym Carolyn Keene. The actual author was ghostwriter Harriet Stratemeyer Adams. The Thirteenth Pearl is the end of the original 56-book series published by Grosset & Dunlap. Subsequent volumes were published by Simon & Schuster. Source: Wikipedia (en)