Harrison, Robert, trans. The Song of Roland. Mentor, New York, 1970. Harrison's introduction is sensible, well-researched, and concise.
Hollier, Denis, ed. A New History of French Literature. Harvard U.P., Cambridge, MA, 1989. The essays "Entering the Date" by John Benton, "The Birth of Medieval Studies" by R. Howard Bloch, and "The Epic" by Joseph J. Duggan give fascinating new perspectives on The Song of Roland. Highly recommended.
Le Gentil, Pierre. Beer, Frances F., trans. The Chanson de Roland. Harvard U.P., Cambridge, MA, 1969. A fine general study.