Art Of War
You will often here references to "The Art of War" when talking to players and developers of Shogun: Totalwar. This book was written by an ancient Chinese strategist named Sun Tzu, and it is a set of principles and rules which will enable a general to achieve victory. These rules and principles have been programmed into the very core of Shogun by the developers, and apply very much to the game. Learning these documents will give you a better understanding of the battlefield and a better chance of victory.

These documents are all provided by Nicholas Novitski, who has written them all up and added annotations of his own. Many thanks to him.