I read this book 35 years ago, and I remembered it with great fondness. I liked the movie, too. Mitchum is great in it. The cover pictured to the left is on the British edition of the book. As you can see, the author is Leonard Schrader. Paul Schrader and Robert Towne wrote the screenplay for the movie.
This is one of those novelizations that expands a lot on the movie script. It's almost 250 pages long, and the print's small. It's essentially the same story, though. Harry Kilmer (Mitchum in the movie) is a retired p.i. who's asked by an old friend to rescue the friend's daughter, who's been kidnapped by the Yakuza. Kilmer doesn't want to take the job because of things that happened in Japan during the American occupation after WWII, but the old friend convinces him. Kilmer finds the girl and rescuse her almost immediately. End of story, right? Of course not. The trouble is only beginning.
Nobody goes up against the Yakuza and wins, or so they say. They just don't know Kilmer. There's a lot of violence in both book and movie, and also a lot about friendship and honor codes. I think I was originally attracted to the book for those reasons and because of the detailed Tokyo setting.
Reading the book again, I realize that while the story and the emotions still hold up for me (and I'm sure the movie would, too), the writing doesn't. "The rosy-fingered sunset slipped into the dark glove of night." I might have admired that 35 years ago, but now, not so much. There are other stylistic quirks that at the time must have appealed to me. They don't now. But the book held my attention and zipped right along, so I can't complain. I expect that I'd now find the movie superior, however. I wonder if it's on DVD.