In the Shadow of the Sword by Tom Holland

In the 6th century AD, the Near East was divided between two venerable empires: the Persian and the Roman. A hundred years on and one had vanished forever, while the other seemed almost finished. Ruling in their place were the Arabs: an upheaval so profound that it spelt, in effect, the end of the ancient world. In the Shadow of the Sword explores how this came about. Spanning from Constantinople to the Arabian desert, and starring some of the most remarkable rulers who ever lived, he tells a story vivid with drama, horror, and startling achievement.