Barnard is a leading Keats scholar, and has published extensively on Restoration literature, the second generation Romantics, textual criticism, and book history. Publications include editions of Congreve's Way of the World (1972), Etherege's Man of Mode (1979), John Keats: the Complete Poems (1973, 3rd ed. 1998),and Keats's Selected Lettters (2014). He has also published Alexander Pope: The Critical Heritage (1973), John Keats (1987), and with D. F. McKenzie co-edited and contributed to The Cambridge History of the Book in Britain, Volume 4, 1557-1695 (2002).
From 1975 to 2010 Barnard was general editor of Longman Annotated English Poets and since 2003 has acted as general editor with David McKitterick and Ian Willison of the Cambridge History of the Book in Britain. He was President of the Bibliographical Society, 2008 to 2010, is a Senior Research Fellow, Institute of English Studies, University of London, and gave the 10th D. F. McKenzie Lecture, English Faculty, Oxford (2005), the John Coffin Memorial Lecture in the History of the Book, Institute of English Studies, London (2010); and was Guest lecturer, First Bicentenary John Keats Conference at Keats House (2014).