Complex sentence: He published the books "Fra Oscarsborg til Hegra" in 1945, on the Norwegian Campaign in southern Norway, and "Usynlige Veier" in 1946, on refugee escorts and courier traffic between Norway and Sweden during the war.
Split into simpler sentences: He published the book "Fra Oscarsborg til Hegra" in 1945, on the Norwegian Campaign in southern Norway. In 1946 he published "Usynlige Veier" ("Invisible paths"), on refugee escorts and courier traffic between Norway and Sweden during the war.