Private detective talented but not physically strong, must be taught to protect himself. Set in 50s, 60s or 70s in U.S.

[Pthah]

Hello people
Please help me to find a book. All I remember about it is that the main hero is a private detective who works in a detective agency. He is really good and talented but he is not very strong physically and his boss had to hire a trainer who would teach the main character to protect himself. The book is rather old. I think it takes place in 60s or 70s. Thanks a lot beforehand!

[Kris]

Pthah, it sounds like this book is for older adults, not young adults (teens).

Can you describe the main case the private detective is working on?

What's the time period and location (country, region, big city/small town)?

[Pthah]

I do not remember unfortunately. I think it takes place in the USA in the middle of 50s or later. That's it

[Kris]

What does the trainer teach him to do (e.g., martial arts, how to use specific weapons)?

What's the tone - gritty crime, mystery, thriller, funny at times, etc.?

[Pthah]

Oh I think martial arts. It was typical private eye kind of story. The fact that the main hero wasn't a typical rough guy who can easily knock out an opponent that what was unusual and attracted my interest, and that is why it is the only thing that I remember!

[Lobstergirl, au gratin]

Pthah, are you still looking for this or did you find it?