The 47th Samurai
This is not a book about ancient samurai warriors in Japan. Instead it is a modern 'thriller' set in modern Japan, but it is about samurai warriors and lots of sword fighting. It is the forth book by Hunter that features Bob Lee "The Nailer" Swaggert, an ex marine corps sniper in the Viet Nam war. I have read all four of them, and this is the least of the four. In fact, I would rank them in order of their publication date, the best being Point of Impact from 1993. It was recently filmed as The Shooter.
This book takes an older, 60ish Bob Lee and send him to Japan in search of revenge. There is no way he can take his sniper rifle into Japan, so he uses swords. In just two weeks! he becomes expert enough to take on swordsmen who have studied for years. And defeat them!
Nothing the least bit realistic here, just lots of cutting and blood. It seems to me as if Hunter made a two week trip to Japan, decided to write a novel set there, learned a lot about swords from research, and then used one of his older characters to be a samurai. Being part of the American gun culture (whether I like it or not), his earlier books about sniper rifles and snipers were much more interesting. I found myself skimming passages that were about sword strokes and sword culture and practice.
If you are interested in thrillers of this type, read Point of Impact.