Nancy Drew 01: The Secret of the Old Clock by Carolyn Keene

Nancy Drew is the classic young sleuth. Her mysteries are exciting for elementary and middlde school readers. There is also a series under the same author called Nancy and the Clue Crew which is best for beginning readers. Books at the first of the series were written in the 1960s, but later books include a more modern feel with computers, cell phones, and current issues.