Lest Darkness Fall Author:L. Sprague de Camp When am I? — Padway asked himself after the lightning-flash knocked him down. — He knew where he was--Rome. He was there to study archaeology, and even though the lightning had left him dazed, he could see the familiar Roman buildings... — But the buildings looked newer...and the crowds in the street were wearing tunics, not suits! And a rich bar... more »nyard smell had replaced the gasoline-and-garlic aroma of modern Rome...
So...when was he? And he was suddenly cold with fear of the answer...« less
Not at great example of this sub-genre, but it is entertaining and more realistic. I checked on Fantastic Fiction and this novel is one of the first three books he wrote, in 1941.