The first book in the series, The Secret of the Old Clock, was written in 1930 and released later that same year. It was quickly followed by 55 more stories, all published under the female pseudonym of "Caroline Keene" — although in fact the books were ghost-written by the same author who wrote The Hardy Boys, Franklin W. Dixon, and his "Stratemeyer Syndicate".
Throughout the series, Nancy is portrayed as a young, rather scatter-brained girl, who always carries a flashlight (or "torch", as the Anglophile Dixon would call it on several occasions), which she uses to light her way through a series of secret underground passageways. Each book typically ended with Nancy's being captured and held prisoner by some type of villain, at which point she would normally be rescued by one of the series' strong male characters, such as Ned Nickersen or Carson Drew.
The amazing success of the series led to several movie deals in the late 1930s, starring child actor Bonita Granville. A third movie was made in 2007, starring Julia Roberts' niece Emma in Granville's role.