Reddit reviews Journey of King Koopa RU: Koop Hidding box (Tell the Koopa) (Volume 1)
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So the King Koopa book is the easiest one to do research on from what I can tell, it's written by an "Anthony Koop Goethe III".
Here's an Amazon link to "Journey" https://www.amazon.com/Journey-King-Koopa-RU-Hidding/dp/1974547213
As you can see, on the second photo there there is a very blurry photo of the back cover of the book which seems to have an author bio on it and even a photo of King Koopa himself. My enhancing abilities are minimal so I can't quite make out what it says but I'm sure someone could do it.
Doing some more digging leads me to another piece of the Koopa saga:
https://www.amazon.com/Koop-Brasi-Story-these-story/dp/1720599173
This one is actually available for purchase (with prime lol), and again seems to have an extensive and detailed back cover. Probably isn't too hard to track down this guy and find out what the deal is.
forgot to add, the "Only The Strong" book is listed as being written by a "Jame Goethe", given that I see Anthony Goethe listed as AJ in some places, would not be surprised if it's the same guy.
update: here's his website https://koppa.webs.com/about-koopa