I'm voting for "The Spiritual Foundations of Aikido" by William Gleason. I found it to be an enlightening mix of spiritual philosophy and interesting techniques which offer some physical explanations of the spiritual concepts. A little confusing the first couple times around (for a noob) especially concerning the applicability of the kotodama in theory and practice. All in all a good read.
Also, "The treatise on the Taia sword" by monk Takuan Sōhō. Interesting piece that blurs the line between action and intention.
Tsunetomo Yamamoto's "Hagakure"
Bruce Lee's "The Tao of Jeet Kune Do"
+1 for Morihei Ueshiba's "The Art of Peace"
+1 for Sun Tzu's "The Art of War"
+1 for Miyamoto Musashi's "The Book of Five Rings"
+1 for Lao Tzu's "Tao Te Ching"
+1 for the PICTURES
in "Aikido and the Dynamic Sphere"
P.S. - If it's not on the list, it's probably because I havent read it yet!