The p.i. narrator is Jame Rehm. In the opening chapter, he's about to marry the girl of his dreams, but the FBI tells him that he's needed for a mission. "The fate of the world" hangs on its success. He's to tell no one, not even his bride-to-be. Rehm evades the Feebies and gets married, but they snatch him up and convince him he has to do the job. Which is where I run into trouble with the plot. There's no reason they have to use Rehm. He has no special talents and he knows no one involved in the plot. The FBI could have used anyone. Even an FBI guy. The rest of the book is the pursuit of certain papers that contain all of Russia's secrets. That's right. All of them. "The fate of the world" is mentioned again and again. Rehm blunders along. There's train travel (a nice bonus) and one good surprise, but that's about it. Nothing lifts the book out of the ordinary. Matt Helm came along a few years later and did this sort of thing a hundred times better.
Herber continued to publish for years, and maybe some of the later books are more to my taste than this one. I'd probably pick one up if I ever ran across it, but I wouldn't go looking for it.