His doctorate thesis from 1932 was a treatment of John Locke. He was Norwegian editor of the three volumes of Den store krigen, published from 1946 to 1949. Among his other works are biographies of Marcus Jacob Monrad, Eilert Sundt and Niels Treschow.
He wrote several books on the history of travelling and tourism, and World World War II memoirs Fra nød til seir.
During the occupation of Norway by Nazi Germany he was a central member of the Norwegian resistance movement. From late 1944 he lived undercover as an active member of Hjemmefrontens Ledelse.