Northrop Frye, The Return to Eden: Five Essays on Milton's Epics (Toronto: Toronto UP, 1965).
Introduction to Paradise Regained, in The Complete Poetry and Essential Prose of John Milton, ed. William Kerrigan, John Rumrich, and Stephen M. Fallon (New York: Modern Library, 2007).
Barbara Lewalski, Milton's Brief Epic: The Genre, Meaning, and Art of Paradise Regained (Providence: Brown UP, 1966).
Susanne Woods, introduction to Paradise Lost & Paradise Regained, published by Signet Classics.