The Complete Paratime — H. Beam Piper

Infinite worlds allowed for infinite evil -- unless the doorway to those worlds was most carefully guarded. The Paratime Police are an elite force of men and women charged with defending a million unsuspecting Earths from their more developed -- and more ruthless -- neighbors in parallel continua. From the original Paratime, which introduced the unceasing struggles of the time-traveling heroes, to Lord Kalvan of Otherwhen, the last Paratime story of a state trooper's conquest of an alternate Earth...The Complete Paratime is a grand science fiction experience.