A determined pioneer in the history of art in Canada, he constantly stressed its need for careful, detailed knowledge of artists, their works, and their cultural setting as a means of full understanding of Canadian traditions. He produced essential aids in his Historical Directory of New Brunswick Newspapers and Periodicals (1961) and Early Painters and Engravers in Canada (1970). Painting in Canada: A History (1966) was the first comprehensive study in its field and the first important art book entirely produced in Canada; a revised edition appeared in 1977. His bent for inquiry and his sensitivity to creative expression combined in Paul Kane's Frontier (1971), which studied Kane's biography and writings in relation to the sketches and paintings documenting his travels. The same skills made his Krieghoff (1979) a definitive study.
In his later years Harper had 2 chief interests. One was the development of the first graduate program in Canadian art history. The other was the study of FOLK ART in Canada: the result was a seminal touring exhibition in 1973 for the National Gallery and A People's Art (1974).