The secret teachings of the Minamoto han became known as the Tosen kyo, the scroll of combat or war. For a long time it was only transmitted within the Minamoto family when a person had achieved menkyo.
The Tosen kyo dates from the 10th -- 11th century (Heian period) and is written by a member of the Oe family (long time teachers in the art of war), teachers of the famous general Minamoto Yoshiie and of Minamoto Yoshimitsu (according to some the founder of Daito Ryu Aiki jujutsu).
The book is about strategy in a classical sense, to succeed in combat the warrior needed knowledge about nature, the origin of life and his place in creation, heaven and earth, Izanagi and Izanami, In and Yo, the Gogyo and Jinrin. The Tosen kyo is as much about Bu as it is about Bun.
Morihei Ueshiba O Sensei quoted from this book and referred to it in some of his writings.
The Tosen kyo is one of those rare classic writings on Budo that one would like to see translated in English.
Fortunately it has been translated into Dutch by Jeroen Bode (Vulcanus 2002).
I have mentioned a few chapters of the Tosen kyo on my weblog; http://aikido-auvergne-kumano.blogspot.fr/