Lest Darkness Fall, by L. Sprague de Camp, paperback novel
“One minute archaeologist Martin pAdway is casually ambling through modern Rome. In an instant he is inexplicably hurtled back through time to 6th century Italy – just before the Dark Ages. With one foot firmly rooted in the 20th Century, and the other planted tentatively in the Gothic era, Padway – now Martinus Paduei, Quaestor – uses his wits and his knowledge to change the course of history.”
1979 printing of 1974 Ballantine edition.