H. Beam Piper 
Lord Kalvan of Otherwhen [EPUB ebook] 

The Paratime Police patrol the vast number of alternate time-dimensions. Their aim is to keep the existence of the alternate Earths a secret and prevent these Earths from mixing and destroying each other.
But the Time Police make mistakes sometimes, and they make a big one when a seemingly ordinary Pennsylvania State Trooper named Calvin Morrison is accidentally transported to a more backward parallel universe. Morrison is soon being hailed as Lord Kalvan and is masterminding a campaign that could blow the whole Paratime secret sky-high!