Curious Notions

Curious Notions is an alternate history novel by Harry Turtledove. It is a part of the Crosstime Traffic series. In Curious Notions, the Central Powers won World War I prior to the United States entering the war. Subsequently, the German Empire invaded and conquered the United States in the 1950s. The story is set 150 years later, in German-occupied San Francisco. The main plot deals with time travelers from our universe establishing an electronics shop in San Francisco, coming under the suspicion of both the German authorities and the Tongs while preventing the Germans from duplicating the time travel technology.