The name of the novel, Yin Yang Agent, may contain many different stories and series, so it is impossible to determine how many books there are in total. If he was referring to a specific Yin Yang agent novel series, it might include the following parts: - The Yin Yang Agent - [The Yin Yang Agent: Yin Yang Eyes] - [The Yin Yang Agent: Ghost Game] - [The Yin-Yang Agent: Onmyoji] - [The Yin Yang Agent: Traversing the Other World] These are just some of the possible options. The exact number of books depends on the chosen chapter and storyline.