Although his early work focused on English religious and intellectual history, Cantor's later scholarly interests were far more diverse, and he found more success writing for a popular audience than he did engaging in more narrowly-focused original research. He did publish one monograph study, based on his graduate thesis, Church, kingship, and lay investiture in England, 1089-1135, which appeared in 1958 and remains an important contribution to the topic of church-state relations in medieval England. Throughout his career, however, Cantor preferred to write on the broad contours of Western history, and on the history of academic medieval studies in Europe and North America, in particular the lives and careers of eminent medievalists. His books generally received mixed reviews in academic journals, but were often popular bestsellers, buoyed by Cantor's fluid, often colloquial, writing style and his lively critiques of persons and ideas, both past and present. Cantor was intellectually conservative and expressed deep skepticism about what he saw as methodological fads, particularly Marxism and postmodernism, but also argued for greater inclusion of women and minorities in traditional historical narratives. In both his best-selling Inventing the Middle Ages and his autobiography, Inventing Norman Cantor, he reflected on his strained relationship over the years with other historians and with academia in general.