The Secret of the Old Mill

The Secret of the Old Mill is Volume 3 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.

This book was written by Leslie McFarlane in 1927 for the Stratemeyer Syndicate, who published it under the pseudonym Franklin W. Dixon. On January 1, 2023, the 1927 version entered into the US Public Domain, due to 2022 having been the book’s 95th year. However, the 1927 version is still under copyright in Canada until January 1, 2048, due to Canada using the author’s life plus 70 years model for works-for-hire. As Leslie McFarlane died in 1977, the last year for Canadian copyright on the 1927 version will be 2047. The 1962 version will enter the US Public Domain on January 1, 2058, and the Canadian Public Domain as late as January 1, 2067 (Alistair Hunter possibly died in 1996). Between 1959 and 1973 the first 38 volumes of this series were systematically revised as part of a project directed by Edward Stratemeyer's daughter Harriet Adams. In 1962, the original version of this book was rewritten in by Alistair Hunter, resulting in two different stories with the same title.