The third book.
secret of the old mill.
"The Secret of the Old Mill" was first published in 1927, and the story is set in the 1920s as well. The exact time frame within the 1920s is not specified in the book.
Barmet Bay/ Bayport
The Secret of the Caves
what did frank fin after he flushed on of the ยจspy pack
The Secret Knowledge Of Organized Lawbreakers (SKoOL) appears in book #46 (the Secret Agent on Flight 101)
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 group UGLI was mentioned in the Hardy Boys book "The Flickering Torch Mystery" (Book #22) and "The Melted Coins" (Book #23).
Mrs. Hardy made chocolate cake for the boys after school in book 1.
Henry Kurt is in book #33 (The Yellow Feather Mystery) in the Hardy Boys series
Hardy boys
The book "The Hardy Boys: The Tower Treasure" by Franklin W. Dixon has 192 pages.
mysteries The Hardy Boys is actually a series of books - all about teen detectives, brothers Frank and Joe Hardy.
The second Hardy Boys mystery book is titled "The House on the Cliff."
In the Hardy Boys book #5, "Hunting for Hidden Gold," the boat is found at the bottom of Bitter Creek by the Hardy Boys during their investigation into the mysterious disappearance of Mr. Kenworthy.