The Secret of the Old Mill is the third volume in the original Hardy Boys series of mystery books for children and teens published by Grosset & Dunlap. The book ranks 86th on Publishers Weekly's All-Time Bestselling Children's Book List for the United States, with 1,467,645 copies sold by 2001. This book is one of the "Original 10", some of the best examples of the Hardy Boys, and Stratemeyer Syndicate, writing.