Way of the Peaceful Warrior. We've got two parts to that title, yes? There's the contradictory-sounding peaceful-warrior bit, and something about a "way."
While we ordinarily think of warriors as tough-talking military commandos (think Arnold Schwarzenegger blasting with a machine gun in Predator), here we have warriors who meditate and don't worry over things. But they're still warriors, the book tells us, whether it's Socrates ninja-ing stick-up criminals (4.375-380), Dan performing gymnastics feats, or simply being happy and truly aware of life.
So that may be a different take on what a "warrior" is than what we're accustomed to.
Now to the "way" part. That's pretty self-explanatory: it means how to go about being a peaceful warrior. Socrates tells Dan, “I have shown you the way of the peaceful warrior, not the way to the peaceful warrior. As long as you tread the way, you are a warrior” (8.20). And of course, the book is basically an instruction manual in the way of peaceful warriorhood.
All in all, it's a straightforward title with no tricks.