For me, I like the following:
The Art of War, Sun-Tzu.
The Book of Five Rings, Musashi Minamoto.
The Life-Giving Sword, Yagyu Munenori.
Warrior's of Medieval Japan, and
The Samurai and the Sacred, by Stephan Turnbell.
The Demon Sermon on Martial Arts, Issai Chozanshi.
The Art of Peace, (which I like for the quotes by Ueshiba).
Koryu Bujutsu, Diane Skoss
The Unfettered Mind, Takuan Soho
"Seek not to follow in the footsteps of the men of old; seek what they sought."