H Beam Pipers Lord Kalvin of Otherwhen
In it a PA state policeman is caught in an alternate timeline transport, and sent to a time line in which the indoeuropeans went east and ended in the americas.
The time line is characterized by the invention and control of gunpowder bya religious order call the House of Styphos (I think)
But Calvin knows the formula, prevent 'little Nell's" kingdom from being over run.
Unfortunately, Piper only wrote a single story, John Carr and Roland Green collaborated in a sequels that aren't too bad and do capture Piper's vision. .