Reddit reviews Kendo: Elements, Rules, and Philosophy (Latitude 20 Books (Paperback))

It's hard to describe. You want the swing to be firm when you strike someone. The goal isn't to smack them with brute strength or not use any force at all. Your strikes must be precise and controlled to allow the judge to determine that you had control over what you were doing and meant to do it if that makes sense. It's all about control and discipline.

There are several books that I'm sure you could find. I had this one while I was growing up and read it a bit. I think it was pretty good, but I can't remember because I was so young.