The Secret of the Old Mill (Hardy Boys, Book 3)

Determined to learn the secret of the old mill, Frank and Joe employ a clever use to gain entrance and become trapped. There they unravel two mysteries, one involving a counterfeiting case and the other, a national security case their father is working on.

This is not the original edition: it was re-written in the 1950's to be more politically correct. The vocabulary and sentence structure are also simpler than the original.