Lord Kalvan of Otherwhen

A State Trooper steps around a tree and finds himself in a another world. Fortunate for him he was a student of military history because the new world he finds himself in, needs him. State Trooper Calvin becomes Lord Kalvin due to his knowledge of early gunpowder and weapons. Classic Science Fiction, multi-dimensional travel into an alternate universe where there is a corrupt gunpowder theocracy just waiting to be broken up, a beautiful princess and lots of black powder weapons. Probably the first SF alternate history story from a historical perspective, Piper beat everyone to this genre, and did it so well at it that it took several decades for any competitors to appear. Good characters, good action, good historical analysis.