Lest Darkness Fall Author:L Sprague de Camp A Step Out of Time — One minute archeologist 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... more »--uses his wits and his knowledge to change the course of history« 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.