A kappa is a mythical creature from Japanese folklore that looks like a cross between a turtle and a monkey. It lives in rivers and ponds and has a hollow space on the top of its head that holds water. It's said that if you bow to a kappa, it will bow back and spill the water on its head, rendering it weak and easy to capture.
In some Japanese novels, the kappa is often used as a symbol of Japan's cultural heritage and the importance of maintaining a connection with nature. These stories often feature a hero who must outsmart or defeat a kappa to save their village or loved ones.
So, a kappa novel is a book that tells a story featuring this mythical creature as a central theme. It's a way of exploring Japanese mythology and culture through the lens of storytelling.