I loved "The secret of the ninjas". It had a better research from Japanese culture than tons of local pulp novels (from Spain) about some US born Kung Fu fighting spy/secret agent soon-meeting-a-Japanese-sensei-with-cheap-tropes-everywhere.

That being a book for kids compared to cheap pulp novels were meant for lowly-educated adults, but I would expect less for preteen books where a perfect mythos depiction wasn't always a thing.

Still, Hollywood movies and some Asian exploitation series weren't much better...