Lest Darkness Fall
Lest Darkness Fall is the title of one of the best and most time tested of the "alternate history" concept novels.
By L Sprague de Camp. Good author.
In it, a modern man, a la Connecticut Yankee in King Arthur's Court, finds himself in ancient Rome,
and sets about preventing the Dark Ages. First he must survive, then he must defeat Belisarius (SF writers all love general Belisarius of Byzantium, for good reason - if you got to rip off the ancients, rip off the best, via the best). Then life gets interesting.
Most of the rest of de Camp's stuff is light classical fantasy. The Incompleat Enchanter is a fun series, playing with literature, most of the rest is distracting but forgettable, he is probably best known for editing and extending Robert Howard's classic "Conan" series.
His other "great book" is the "Ancient Engineers". In it he considers how ancient civilizations could have built their great works, including a history of engineering and great engineering devices. The book is dated, some of the history is now suspect and there are more rigorous and scholarly works on the actual likely mechanics of ancient engineering.
But de Camp did his homework and is a good writer, and modulo it being dated and spotty, it is well worth a read.
Oh - on the topic - people should read "Household Gods" - by Turtledove and Tarr.
It puts a slightly irritating LA lawyer in a border town of the Roman Empire, during Marcus Aurelius' time if I recall.
Not quite so romantic and heroic. More a struggle for survival and a coming of age story.