As a child, I loved these books! And in the series, this and the Invisible Dog were my favourites! First, let me assure you there is nothing supernatural in the series (SPOILER: The boy is NOT a werefrog!)
I know E W Hildick continued writing the series until well into the 1990s, but as I was born in the mid 70s, the books I read as a child were the earlier ones; before most of the mysteries started involving real crimes. This type of faux crime excited my young mind, as the "mysteries" were things I could relate to as opposed to bank robbers, industrial spies, and (later in the series) time travel and the like. Sure, those were good fun (even reading as a teen and adult), but for tween (and younger) me, the thought that I might be in a "detective organization" of kids and find such incidents in my own neighbourhood was thrilling!
I like how Hildick doesn't baby the books, and that everything really does proceed logically, as the kids solve the mysteries with intelligence and talent. I think this book marks the first time, the gang receives an adult client. The only thing I didn't like was that they basically weren't paid at the end. This seems a common theme, (even in "big boy" books like the Three Investigators!). I understand they donated their earnings towards a cage construction (read the book to learn why), but Mrs Kranz (the client) could easily have afforded to buy the materials for the cage, and still paid the kids! I always felt they got the short end of the stick there, especially as it was such a generous wage! ($10/day, which is a fortune for a 10 year old in 1979! My allowance was $3/week for instance)