Lest Darkness FallBy L Sprague de Camp
Martin Padaway was a smart enough young man, with a scientific education, but no universal genius. He had the misfortune to be suddenly dropped back into time, and a very alarming time at that-sinth century Rome, when the Goths ruled Italy, and civilization in the west was collapsing. To make a living, and to try and shore up civilization, Padaway undertook to introduce inventions such as gunpowder, clocks, and printing. Some worked and some didn't, often with dramatic and hilarious results.