The best texts to use are Paradise Lost, ed. Alastair Fowler, and Complete Shorter Poems, ed. John Carey (both in the Longman Annotated English Poets series; 1968, 1971; revised, 1997 and 1998). The Penguin Paradise Lost, ed. Christopher Ricks, is cheaper and you can put it in your pocket.
John Carey, Milton (1969).
David Daiches, Milton (1957).
T.S. Eliot, ‘Milton I’ and ‘Milton II’; in On Poetry and Poets (1957). The second is in Thorpe, below.
Barbara Everett, ‘The End of the Big Names: Milton’s Epic Catalogues’; in English Renaissance Studies, ed., John Carey (1980); reprinted in Everett, Poets in their Time (1987).
William Empson, Milton’s God (1961).
Stanley Fish, Surprised By Sin: The Reader in Paradise Lost (1967).
-- ‘Question and Answer in Samson Agonistes’, Critical Quarterly 11 (1969).
Geoffrey Hartman, ‘Milton’s Counterplot’, Journal of English Literary History 25 (1958); reprinted in Hartman’s Beyond Formalism (1970).
Christopher Hill, Milton and the English Revolution (1977).
Samuel Johnson, ‘Milton’, in the Lives of the Poets (1777); in Thorpe, below.
B.K. Lewalski, ‘Milton on Women - Yet Once More’, Milton Studies 6 (1974).
-- Milton’s Brief Epic: Paradise Regained (1966).
C.S. Lewis, A Preface to Paradise Lost (1942).
Louis Martz, Poet of Exile: A Study of Milton’s Poetry (1980).
M.A. Radzinowicz, ‘Milton, the Bible, and Paradise Regained’; in Dennis Danielson (ed), The Cambridge Companion to Milton (1989).
Christopher Ricks, Milton’s Grand Style (1963).
-- (ed.), The Sphere History of English Literature: English Poetry and Prose 1540-1674 (1986): three Miltonic chapters.
James Thorpe (ed.), Milton’s Criticism: Selections from Four Centuries (1951).