He specialized in the history of education, and wrote Norsk lærerutdanning fra 1814 til i dag (1959), Lærerutdanningen ved Universitetet i Oslo fra 1814 til i dag (1964), Klassisisme og realisme.
In 1977 he completed Aslak Torjusson's unfinished work Den norske folkehøgskulen.
Opphav og grunnlag, a standard work about Norwegian folk high schools.
Dahl also wrote several books and newspaper pieces pertaining to his native Rjukan and Tinn.
Professor Dahl was a fellow of the Norwegian Academy of Science and Letters from 1975, and retired in 1991.