John Haldon 
Byzantium at War AD 600-1453 [PDF ebook] 

Byzantium survived for 800 years, yet its dominions and power fluctuated dramatically during that time. In this book, John Haldon tells the full story of the Byzantine Empire–from the days when it was barely clinging to survival, to the age when its fabulous wealth attracted Viking mercenaries and Asian nomad warriors, to its armies whose very appearance on the battlefield was enough to bring enemies to terms. In 1453, the last emperor of Byzantium, Constantine XII, died fighting on the ramparts, bringing to a romantic end the glorious history of this legendary empire.