Paratime — H. Beam Piper

The Paratime series of several stories dealing with an advanced civilization that is able to travel between parallel universes with alternate histories. It uses that ability to trade for goods and services that their exhausted Earth cannot provide. Stories included are: He Walked Around the Horses, Police Operation, Last Enemy, Temple Trouble, Genesis, Time Crime. Henry Beam Piper was an American science fiction author best known for his extensive Terro-Human Future History series of stories and a shorter series of Paratime alternate history tales.