 Paratime series

The Paratime series written by H. Beam Piper and subsequently by John F. Carr consists of several short stories, one novella, and one novel; they deal 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 their own, exhausted Earth cannot provide.Specifically, the Paratime series deals with the Paratime Police, the organization that protects the secret of paratime travel.