The Hardy Boys book that mentions the Klack Agency is The Phantom Freighter (book 26) originally published January 1, 1947. It was credited to Leslie Mc Farlane but was actually written by his wife, Amy
Henry Kurt is in book #33 (The Yellow Feather Mystery) in the Hardy Boys series
According the the Hardy Boys Wiki she makes her 1st appearance on page 8 of Book #45 (the Mystery of the Spiral Bridge)
The group UGLI was mentioned in the Hardy Boys book "The Flickering Torch Mystery" (Book #22) and "The Melted Coins" (Book #23).
#1 The Tower Treasure was adapted for the Applegate TV season, and he is mentioned in While the Clock Ticked
The writer of the hardy boys is Franklin W. Dixon.
Yes the hardy boys are back together
Yes, the Hardy Boys, Frank and Joe Hardy, do have a brother named Chet Hardy. He is an occasional character who assists them in their detective adventures.
The last Hardy Boys book with Callie Shaw is "The Hardy Boys #163: A Taste for Terror" which was published in 1995. Callie Shaw is a recurring character in the series as Joe Hardy's girlfriend.
The Hardy Boys Detective Handbook was created in 1959.
There are 66 Hardy Boys books in all.
The Hardy Boys television show began in 1977. The Hardy Boys shared the program with Nancy Drew for the first two seasons, but in the third and final season the entire show was devoted to the Hardy Boys.
The Secret Knowledge Of Organized Lawbreakers (SKoOL) appears in book #46 (the Secret Agent on Flight 101)