Charles Oman 
The Art of War in the Middle Ages [EPUB ebook] 
Military History of Medieval Europe (378-1515): The Transition From Roman to Medieval Forms in War, the Byzantines and Their Enemies, the English and Their Enemies, Feudal Cavalry

The Art of War has been very simply defined as 'the art which enables any commander to worst the forces opposed to him.’ It is therefore conversant with an enormous variety of subjects: Strategy and Tactics are but two of the more important of its branches. Besides dealing with discipline, organization, and armament, it is bound to investigate every means which can be adapted to increase the physical or moral efficiency of an army.
Contents:
The Transition from Roman to Mediæval Forms in War – A.D. 378–582.
The Early Middle Ages – A.D. 476–1066–81.
The Byzantines and their Enemies – A.D. 582–1071.
The Supremacy of Feudal Cavalry – A.D. 1066–1346.
The Swiss – A.D. 1315–1515.
The English and their Enemies – A.D. 1272–1485.