T.E. Lawrence's Seven Pillars of Wisdom. He assisted in developing many of the insurgent tactics now used by many of the groups in Iraq (notably the IED, economy of force, blending into the surrounding territories, de-centralization of command, etc). He also makes comments on the organizational culture of the British Army that are actually quite similar to the organizational culture of the US Army in the War on Terror, which followed a long period of peacetime.