Westberg promotes the use of historical perspectives and methods within the field of educational research, and has paid special attention to the history of primary schooling and early childhood care and education (ECCE). He has a particular interest in questions regarding the how's and why's of educational expansion, not the least the rise of mass schooling during the 19th century and the expansion of ECCE since the 1960s. As a result, he has published monographs, edited books, textbooks and journals articles covering issues relating to the social and economic history of education; school finance; material culture, school architecture; teachers, teachers' salary and livelihoods, discipline and governmentalities; educational borrowing and educational transfer.

Westberg has published more than 60 book chapters and articles, the latter in journals including History of Education, Paedagogica Historica, Scandinavian Journal of History and IJHE Bildungsgeschichte, and been editor and co-editor of more than 11 edited books and special issues, including the text book Utbildningens historia - en introduktion [History of Education - An Introduction] (3 ed. 2019), and School Acts and the Rise of Mass Schooling (Palgrave Macmillan, 2019). In addition to his dissertation, he has also written two monographs: Att bygga ett skolväsende (Nordic Academic Press, 2014, 445 pp.) And  Funding the Rise of Mass Schooling (Palgrave Macmillan, 2017, 242 pp).