The Paratime series written by H. Beam Piper and subsequently by John F. Carr consists of several short stories, one novella, and one novel, all but one of which were originally published in Astounding Science Fiction under the editorship of John W. Campbell. The series deals with an advanced civilization that is able to travel between parallel universes with alternate histories, and uses that ability to trade for goods and services that its own, exhausted Earth cannot provide. The protagonists of the stories are the Paratime Police, the organization that protects the secret of paratime travel. Source: Wikipedia (en)