The first three titles were published in 1927, and were an immediate success: by mid-1929, more than 115,000 books had been sold.
In each novel, the Hardy Boys are constantly involved in adventure and action.
The original "Hardy Boys Mystery Stories" series ended in 2005.
A new series, Undercover Brothers, was launched the same year, featuring updated versions of the characters who narrate their adventures in the first person.
The Hardy Boys, Frank and Joe Hardy, are fictional characters who appear in several mystery series for children and teens.
The characters were created by American writer Edward Stratemeyer, the founder of book-packaging firm Stratemeyer Syndicate; the books themselves are written by ghostwriters under the collective pseudonym Franklin W. The Hardy Boys have evolved since their debut in 1927.
Undercover Brothers ended in 2012 and was replaced in 2013 by The Hardy Boys Adventures, also narrated in the first person.