I’m looking for a book I read as a child in the 1960s. My clues are very vague (sorry!), but here goes:
The book was a middle-grade novel about two girls who play at being detectives. They follow people around and write in their notebooks. I thought the title was The Key House Mystery or The Key-House Mystery (with a hyphen), but I haven’t found it on sites I’ve tried.
Unfortunately, that’s all I remember. I don’t know the color of the cover, whether there were illustrations, or when it was published.
The book definitely wasn’t Harriet the Spy.