Keegan's Face of Battle describes of the life of the man on the front lines and is  both shocking and compelling. Covering the battles of Agincourt, Waterloo and the Somme (they all took place within a short distance of each other), he describes the similarities and differences of war thru the ages. I've anxiously awaited each of his following books and have never been disappointed. In Price of Admiralty, he does for naval warfare what he did for the infantry in Face of Battle. Mask of Command studies great commanders throughout history (including Grant and Hitler with Wellington and Alexander begs to be debated and studied).

 A History of Warfare returns to earlier subjects with a commanding description of war thru the centuries. For similar treatments, there's Ferrill's Origins of War.

War in the Holy Land

The Cambridge Illustrated Atlas of Warfare : The Middle Ages, 768-1487
The Cambridge Illustrated Atlas of Warfare : Renaissance to Revolution, 1492-1792

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