well ok when i read these it was in three differnt books. lord kalvin of otherwhen, little fuzzy and Paratime.
each a different type of sci fi.
Paratime is a set of stories involving Verkan Val a paratime police officer. it seems on the main "prime" time line human science is extremely advanced and they have developed the ability to travel in time. However they can not travel forward and backward, but parallel travel across different time lines is possible.
so in some cases the world developed similar to the prime timeline however they have not discovered the secret of paratime travel. in other cases the world is undeveloped and there is no humans at all.
it is the job of verkan val and his paratime police to make sure these time lines are not exploited.
in lord Kalvin a state trooper from 1960s pennsylvania is hunting a fugitive when he is accidentally kidnapped by a time car run by a paratime cop. kalvin shoots the cop and rolls out of the car into another world, well another time line anyway. he is still in pennsylvania, and its still 1960, but the people are different. thier tech level is that of the 1650s. there is a series of religions set up all fighting a war against the people that adopt kalvin. he, knowing the secret of gunpowder, breaks the gunpowder monopoly of styphon house, the main enemy.
this has a sequel called the great kings war, however the copy my father had was lost before i could read it and i have never seen a copy anywhere else.
little fuzzy i read a long long time ago, and only once. i dont recall liking it much, at least not as much as lord kalvin