Charles Oman 
The Art of War in the Middle Ages: Military History of Europe (378-1515) [EPUB ebook] 

The Art of War has been very simply defined as ‘the art which enables any commander to worst the forces opposed to him.’ This book shows the evolution of European warfare throughout the middle ages from Roman legions to the first use of fire arms:
The Transition from Roman to Medieval 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.