4.0

A Japanese counterpart to Sun Tzu's The Art of War. A fair amount of this one isn't relevant to most modern life, as it has to do with specific techniques of dueling with a sword, but some of that can be generalized to conflict in general. Also, although it mentions in several places that the same principles apply to leading one group in conflict with another, Sun Tzu's book is more concrete in that regard. Still, a lot of this is widely applicable to human conflict in general.
This edition comes with a bonus, a translation (also by Thomas Cleary) of another ancient Japanese military text, The Book of Family Traditions on the Art of War by Yagyū Munemori. Interesting, because some of the ideas in the two books here agree and some don't, though those are usually in regard to swordfighting technique.
If I was going to hand someone half a dozen books to read to start a self-education in basic military science, this would be one (along with Sun Tzu, Liddell Hart, Machiavelli, and a couple of others.)