L. Sprague deCamp 
Lest Darkness Fall [EPUB ebook] 

Against the Fall of Night The Roman Empire had spread order, knowledge, and civilisation throughout the ancient world. When Rome fell, the light of reason flickered out across the Empire. The Dark Ages had begun; they would last a thousand years. Could a man from the 20th century prevent the fall of Rome? When lightening struck and he was hurled backward into the sixth century, the question became anything but academic to Martin Padway, but even forearmed with a knowledge of 20th century technology and of events to come, what could one man do? But Padway must try, lest darkness fall.