Mysteries might be solved by forensics or psychology. This book, set in 16th-century Japan is heavy on psychology, culture, and superstition. Since Koga Yajiro was poisoned, all the women were automatically suspects, since “poison is a woman’s art.” This included both Iga and Koga woman, whether they were servants or ninjas, with high or low positions.
The investigation depended heavily on analyzing formal relationships. Since everyone was a spy or negotiator, there was much speculation on who could be trusted. As might be expected of a story set in Japan, there was a lot of bowing, kneeling, apologizing, and talk of honor. It seemed like any minor insult could lead to a fight to the death. Everyone was armed.
As the number of deaths increased, the number of suspects decreased. In the end, Hiro Hattori identified the killer and the killer, full of arrogance, confessed and added to the incriminating circumstances.
If you are looking for a historical murder mystery set in Japan, this book (Shinobi Mystery #5) is perfect. However, you should also look at Dreaming Spies by Laurie King.
For more on Dreaming Spies: https://1book42day.blogspot.com/2018/11/dreaming-spies-by-laurie-r-king.html
Check out https://amazon.com/shop/influencer-20171115075 for book recommendations.