He was well known for his book, The Indo-Greeks,[1][2] published by Clarendon Press in 1957, in which he discussed the thesis of British historian Sir William Woodthorpe Tarn.

In 1987, Narain was granted early retirement from the University of Wisconsin–Madison and returned to India to found the Bhikkhu J. Kashyap Institute of Buddhist and Asian Studies of which he was the first Director.

He published over one hundred articles and reviews on topics related to history, polity, art, archaeology, anthropology, iconography, epigraphy and palaeography, numismatics and religion in various national and international journals.

He continued working on the history of these people, their movements and interactions in the context of South and Central Asia.

At the time of his death, he was working on a multi-volume project entitled "From Kurush (Cyrus) to Kanishka", several volumes of which were ready for publication.