Henri Pirenne 
Mohammed and Charlemagne [PDF ebook] 

This posthumous work of the renowned scholar Henri Pirenne (originally published in 1939) offered a new and decisive explanation of the evolution of Europe from the time of Constantine to that of Charlemagne. His revolutionary ideas overthrew many of the most cherished conceptions concerning the Middle Ages: namely that "the Germanic invasions destroyed neither the Mediterranean unity of the ancient world, nor the essential features of Roman culture" and that "the cause of the break with the tradition of antiquity was the advance of Islam…"