The Tower Treasure,
The House on the Cliff, and
The Secret of the Old Mill were published in 1927. They were the "breeder" for the Hardy Boys series from the Stratemeyer Syndicate as published by Grosset & Dunlap. The actual author was Leslie McFarlane, who was paid a mere $100 for each book, which he wrote from outlines furnished by Edward Stratemeyer himself. Stratemeyer died in 1930, and McFarlane died in 1979.
This article is incomplete.