Try these
Bliss, A. J. Sir Orfeo. Oxford: Oxford University Press. 1966.
Brouland, Marie-Therese. Le Substrat celtique du lai breton anglais : Sir Orfeo. Paris: Didier Erudition. 1990.
Shuldham-Shaw, Patrick, The Ballad King Orfeo. In: Scottish Studie 20: 124*26. 1976.
Sisam, Kenneth, Sir Orfeo. In: Fourteenth Century Verse and Prose. Oxford: Oxford University Press. 1921.
Tolkien, J.R.R. , Sir Orfeo. In: Sir Gawain and the Green Knight, Pearl, Sir Orfeo. Translated by J.R.R. Tolkien. New York, Ballantine, 2003.
JC