A regular contributor to Astounding Science Fiction throughout the influential John W. Campbell era, Clifford Donald Simak produced a body of highly regarded work, winning the Nebula and three Hugo Awards, and is best known for his story suite of future histories: City (available as an SF Masterworks paperback). We also have an omnibus collecting three novels that explore his favoured theme of a depopulated future: Time is the Simplest Thing, A Choice of Gods and the Hugo Award-winning Way Station.
But as ever, for the full story, you need to consult the SFE:
(1904-1988) US writer whose primary occupation 1929-1976 was newspaper work, and who worked full-time for the Minneapolis Star from 1939 until his retirement, when he became a full-time writer of sf, some years past his early prime. He was in fact a prolific and increasingly popular sf figure – after some stories in the early 1930s – from the true beginning of his career in 1938, remaining prolific from that date until his death. His first published stories, beginning with “The World of the Red Sun” for Wonder Stories in December 1931, were less individual than his later work; significantly, however, that first tale deals with Time Travel, which became his favourite sf device for the importation of Aliens into rural Wisconsin, always his favourite venue. Other early work of interest included “The Voice in the Void” (Spring 1932 Wonder Stories Quarterly), about the desecration of a sacred tomb on Mars which possibly contains the relics of a Messiah from Earth; “Hellhounds of the Cosmos” (June 1932 Astounding), in which defenders of Earth who, in order to fight a Monster in another Dimension, combine into a gestalt; and The Creator (March/April 1935 Marvel Tales; 1946 chap; exp with critical commentaries 1981 chap), in which humans and other races travel by Time Machine in order to combat the creator of the universe, who has become bored with his/her handiwork . . .
You can read Clifford D. Simak’s full entry at The Encyclopedia of Science Fiction, and find his books via his Author page at the SF Gateway website.