 At first, as in the 1939 serial novel "Cosmic Engineers", he wrote in the tradition of the earlier "super science" subgenre that E. E. "Doc" Smith perfected, but he soon developed his own style, which is usually described as gentle and pastoral.During this period, Simak also published a number of war and western stories in pulp magazines.His best-known book may be "City", a fix-up novel based on short stories with a common theme of mankind's eventual exodus from Earth.