His first work Forsøg til de nordiske landes, særdeles Norges, gamle Geografi (Inquiry into the ancient geography of the Nordic countries, particularly that of Norway), was published in 1751.

He published Om de gamle Grækeres og Romeres Kundskab om de nordiske Lande (About the Ancient Greek and Roman knowledge of the Nordic countries) in 1769, which became a prelude to his main work, Norges Riiges Historie (The History of the Kingdom of Norway), the first volume of three that were planned was published in 1771.

[4] As part of his research to support later volumes of this work, he set off on an extended journey to collect data in Norway.

The result of these journeys was his famous travel journal Reise som giennem en Deel af Norge i de Aar 1773, 1774, 1775 paa Hans Majestets Kongens Bekostning (Travel through a part of Norway in the years of 1773, 1774, 1775 at His Majesty the King's Expense), of which only the first two volumes were published in 1778.

Later his death, a sizable portion of his personal book collection would be donated to the Gunnerus Library at the University of Trondheim.