He received a doctorate in English Romanticism from the University of Salzburg and has published more than one hundred articles and books on subjects including the Bible, eighteenth and nineteenth century novels, especially the works of Sir Walter Scott, and 19th-century music.
Letellier is particularly noted for scholarship on the life and works of the composer Giacomo Meyerbeer. Letellier's four-volume translation of the composer's diaries has been cited as "the most important work on the composer to be published in English to date". He has also published several studies of the composer's operas and other works which have played an important part in the revaluation of Meyerbeer, the most popular composer of the 19th century, whose works fell into almost complete neglect in the 20th but are now being rediscovered.
As editor and translator:
- Meyerbeer, Giacomo; Letellier, Robert (editor and translator) (1999–2004). The Diaries of Giacomo Meyerbeer (4 vols.). Madison and London: Farliegh Dickinson University Press.
- The English Novel, 1660-1700: An Annotated Bibliography, Greenwood Publishing Group 1997. ISBN 978-0313303685
- The Operas of Giacomo Meyerbeer, Madison, New Jersey: Fairleigh Dickinson University Press 2006. ISBN 978-0-8386-4093-7.
- An Introduction to the Dramatic Works of Giacomo Meyerbeer: Operas, Ballets, Cantatas, Plays, Routledge 2008. ISBN 978-0754660392
- Daniel-François-Esprit Auber: The Man and His Music, Cambridge Scholars Publishing 2010. ISBN 978-1-4438-2597-9.
- Meyerbeer's Les Huguenots: An Evangel of Religion and Love, Cambridge Scholars Publishing 2014.ISBN 978-1-4438-5666-9.
- "Dr Robert Letellier". University of Cambridge, Institute of Continuing Education. Retrieved 31 August 2018.
- "Giacomo Meyerbeer: Jephtas Gelübde (Jephtha's Vow)". book.google.com. Retrieved 31 August 2018.
- Armstrong, Alan (December 2000). "The Diaries of Giacomo Meyerbeer, vol. 1, 1791-1839 (review)". Music Library Association. 57 (2): 393-4.
- Schmid, Rebecca (25 August 2015). "Restoring the Legacy of a Composer". New York Times. Retrieved 1 September 2018.