Wikipedia
The Hardy Boys teamed up with Nancy Drew in this now-discontinued 36 volume series of paperbacks. This is not the first time the trio have crossed paths; earlier crossovers include the 1970s TV series, the novelization of one of the TV episodes (The Hardy Boys and Nancy Drew Meet Dracula), two SuperSleuth books (each book contains 7 short stories), Campfire Stories (a book of 7 short stories in celebration of the anniversary of the Campfire organization), and the Be-A-Detective series (6 choose-your-own adventure books).
The series is based on the Nancy Drew Files and the Hardy Boys Casefiles universes, as evidenced by Shock Waves, in which the melted keys from the car bombing that killed Iola Morton in the first Hardy Boys Casefile, are mentioned. The books are rumored to have been written to introduce Nancy Drew readers to the world of the Hardy Boys, as the books are written under the Keene pseudonym and are told mainly from Nancy's view.
An attraction between Frank Hardy and Nancy Drew was mentioned throughout the series. The books also hinted at the possibility of a romance between Joe Hardy and Bess Marvin; however, neither pairing was given a final closure.