Seeking 1970s YA mystery. Protagonist delivers newspapers and in a subscription sales contest. Has a genius inventor friend (including retractable staircase). Mysterious house with strange noises. Turns out to be a family of circus performers who buy a lot of newspaper subscriptions.
Stump the Bookseller
New format, same book stumper solving.
I’m not completely sure, although it sounds like it might be part of certain boys’ mystery series from around that time or in the 1960s, like:
The Marvelous Inventions of Alvin Fernald
https://www.goodreads.com/book/show/1039249.The_Marvelous_Inventions_of_Alvin_Fernald
The Brains Benton Mysteries
https://www.goodreads.com/book/show/2311878.The_Case_of_the_Missing_Message
The Mad Scientists’ Club
https://www.goodreads.com/book/show/222127.The_Mad_Scientists_Club
Hope that helps!
Brains Benton – Case of the Missing Message