Oh, that!
Well, it's actually debated, but originally, Wonder Boy was supposed to be Wonder Book, but if I remember correctly, it was Ryuichi who regretted not naming Wonder Boy "Wonder Book" later on, and I made this in homage to the unused name!
I'll have to read up on the facts, but that's what I remember him saying, you can join the Monster World discord for more information, it's real neat there! I hope this clears things up!