The legend of the golden key
Tom McCaughren
Four young boys find adventure involving ghosts and buried treasure when they set out to discover the secret behind the old legend of the golden key.

by Stephen Mooser (1988)
Horace and Mildred Flopbottom hire the young professionals of the Treasure Hounds organization to find out where Aunt Henrietta stashed her fortune before she died. Also includes two descriptions of actual treasures which have never been found.
Tom McCaughren
Four young boys find adventure involving ghosts and buried treasure when they set out to discover the secret behind the old legend of the golden key.
Franklin W. Dixon
Frank and Joe Hardy are called upon to find a wealthy industrialists's son who has mysteriously disappeared. Did Graham Retson flee from his home because he was at odds with his father, or was he kidnapped? Finding the answer turns into something more dangerous and larger in scope than Frank and Joe ever bargained for. The only clue in the case leads the two young detectives to South America, where the intrigues of evil adversaries almost cost them their lives. Involved in this complex mystery are a fierce-looking monkey trained to be a killer, a school of voracious, man-eating piranhas, a ruthless gang of passport thieves, and a cleverly operated change-your-identity mill for criminals. Only through their courage and perseverance do the Hardys solve one of the toughest cases they have ever tackled. - Flyleaf.
John Peterson
Weasel is after something, and he's going to trick Flying Squirrel to get it. But Flying Squirrel has a friend, Mr. Bat, who isn't going to sit around and let Weasel get away with it!
Donald J. Sobol
Leroy Brown is back in the next six books in the Encyclopedia Brown series. As Idavilles ten-year-old star detective, Encyclopedia has an uncanny knack for trivia. With his unconventional knowledge, he solves mysteries for the neighborhood kids through his own detective agency. But his dad also happens to be the chief of the Idaville police department, and every night around the dinner table, Encyclopedia helps him solve some of the most baffling crimes. With ten confounding mysteries in each book, not only does Encyclopedia have a chance to solve them, but readers are given all the clues as well and can chime in with their own solutions. Interactive and funits classic Encyclopedia Brown!