I thought for sure the solution would be to make the letters into a Sator Square and then find the common theme between those words to find the solution word. The format for this puzzle felt a bit random. I hesitated using clues because I didn't want that "oh, duh!" moment, but I'm glad I did. Not sure I would have figured this one out without them. (With that said, I am /loving/ this book, so I don't mean to sound overly critical.)
The premise of the Sator square is a red herring.The letter frequencies in M’s make a symmetrical square impossible. That’s so unsatisfying! Methinks you have the basis for a much better puzzle, but this one felt like you phone it in, M.