A novel by L. Sprague de Camp, which originally appeared as a novelette in 1939. It launched the alternate history genre, setting a very high bar with its historical accuracy and attention to detail. The story reads as well today as when it was written.
The protagonist is visiting Italy where an unexpected and largely unexplained incident of spontaneous time travel deposits him just before the onset of the Dark Ages. Armed only with his knowledge of the period, he attempts to restructure society to prevent the onrushing collapse which he knows was/will be coming in his own timeline.
Fortunately he speaks Latin, which gives him a leg up on most of us. ALl does not go smoothly, and he makes a number of serious errors which make the story more engaging, though he overcomes most of them in the course of the book.