Yagyu Munenori has a philosophical text on The Sword and The Mind, strangely titled The Sword and The Mind.
I guess it depends if you're into that kind of stuff, mixing beliefs and actions. It really doesn't have much to do with exact martial arts; In fact, many places he explicity states the techniques he is trying to exemplify can only be taught verbally, so he doesn't even try. I guess it was written when paper was much more valuable than it is now, or he honestly feels it is something that cannot be described in text.
Another book I've enjoyed reading is John Keegan's The Face of Battle.
It has to do more with the evolution of the human aspect of battle, how soldiers react in the various situations battle has evolved into. The book is divided between three battles, Agincourt, Waterloo, and the Somme, all of which occur about 600 years apart but each fought within about twenty miles of each other on the same typography. It has really helped me develop the concept that modern battles have evolved into controlled skirmishing, that the average man no longer fights with honor but with fear and uncertainty.