The standard text is the Collected Works, known as the ‘Bollingen Edition’ (Routledge/Princeton):
Aids to Reflection (Collected Works IX), ed. John Beer (1993).
Biographia Literaria or Biographical Sketches of My Literary Life and Opinions (Collected Works VII), ed. James Engell and W. Jackson Bate (2 vols.; 1983).
On the Constitution of the Church and State (Collected Works X), ed. John Colmer (1976).
Essays on his Times (Collected Works III), ed. David V, Erdman (3 vols.; 1969).
Shorter Works and Fragments (Collected Works XI), ed. H.J. Jackson and J.R. de J. Jackson (2 vols; 1995).
The Friend (Collected Works IV), ed. Barbara E. Rooke (2 vols.; 1969).
Lay Sermons (Collected Works VI), ed. R.J. White (1972).
Lectures 1795 On Politics and Religion (Collected Works I), ed. Lewis Patton and Peter Mann (1971).
Lectures 1808-1819 On Literature (Collected Works V), ed. R.A. Foakes (2 vols.; 1987).
Logic (Collected Works XIII), ed. J.R. de J. Jackson (1981).
Lectures 1818-1819: On the History of Philosophy (Collected Works VIII), ed. J.R. de J. Jackson (2 vols.; 2000).
Marginalia vols. 1-6 (Collected Works XII.1-6), ed. Heather Jackson and George Whalley (1980-2001).
Poetical Works. Part One (Collected Works, XVI.1) Poems (Reading Text), ed. J.C.C. Mays (2 vols.; 2001).
Poetical Works. Part Two (Collected Works, XVI.2) Poems (Variorum Text), ed. J.C.C. Mays (2 vols.; 2001).
Poetical Works. Part Three. (Collected Works, XVI.3). Plays, ed. J.C.C. Mays and J. Crick (2 vols., 2001).
Table Talk Recorded by Henry Nelson Coleridge (and John Taylor Coleridge) (Collected Works XIV), ed. Carl Woodring (2 vols.; 1990).
Other important volumes:
The Complete Poetical Works of Samuel Taylor Coleridge […], ed. Ernest Hartley Coleridge (2 vols.; Oxford, 1912).
Biographia Literaria […] with his aesthetical essays, ed. J. Shawcross (2 vols.; Oxford, 1907).
The Collected Letters of Samuel Taylor Coleridge, ed. Earl Leslie Griggs (6 vols.; Oxford, 1956-71).
The Notebooks of Samuel Taylor Coleridge, vols. I-V, ed. Kathleen Coburn et al (each in two parts; 1957-2001).
Shakespearean Criticism, ed. Thomas Middleton Raysor (2 vols.; 1960; repr., 1967).
John Beer’s edition of Coleridge’s Poems (new Everyman hardback edition, 2000) will contain everything you need. (NB: the Everyman paperback is a slightly different text, still good, but not as good as the brand new hardback, a real bargain.)
‘Effusions’ no. 20 (‘To the Author of The Robbers [Schiller]’, 1794), no.33 (‘To a Young Ass’, 1794), no.35 (‘The Eolian Harp’, 1795); (take a deep breath) ‘Religious Musings’ (1794-6); ‘Reflection on Having Left a Place of Retirement’ (1795); ‘Sonnet: To the River Otter’ (?1793-?1796); ‘The Foster-Mother’s Tale’ (1797), The Dungeon (1797) - both from his play, Osorio; The Rime of the Ancyent Marinere (1797-8); ‘The Nightingale. A Conversational Poem’ (1798); ‘This Lime-Tree Bower my Prison’ (1797); ‘Kubla Khan’ (1797); ‘Christabel’ (1797-1800); ‘Frost at Midnight’ (1798), ‘The Mad Monk’ (1800); ‘Love’ (1799); ‘Letter to Sara Hutchinson’ (1802) and its revised version, ‘Dejection: An Ode’ (1802); ‘Hymn Before Sunrise’ (1802); ‘The Pains of Sleep’ (1803); ‘To William Wordsworth’ (1807); ‘On Donne’s First Poem’-‘Limbo’-‘Ne Plus Ultra’ (1811); ‘A Character’ (1819); ‘To Nature’ (1820?); ‘Work Without Hope’ (1825); ‘The Pang More Sharp than All’ (?); ‘Duty Surviving Self-Love’ (1826); ‘The Garden of Boccaccio’ (1828); ‘Self-Knowledge’ (1834); ‘Epitaph’ (1833).
Biographia Literaria (1817) is his greatest completed prose work, a piece of literary criticism in general and of Wordsworth in particular, and the nearest he comes to a comprehensive statement of his philosophy of literature and its relationship to his philosophy of mind. It can seem forbidding to start with, but it is not as obscure as it looks. The old Oxford edition by Shawcross (1907) has now been superceded by the Bollingen edition of the book, edited by Walter Jackson Bate and James Engell (2 vols., 1983), not available as a paperback. Both editions have useful, friendly introductions. To begin with, I’d concentrate on chapters 4 and 13 from volume one, and chapters 14-22 from volume two - which includes his criticism of Wordsworth.)
Coleridge’s criticism of Shakespeare and Milton and some others is mostly to be found in the Bollingen edition of Lectures 1808-1819 on Literature, ed. R.A. Foakes (2 vols., 1987), which comprises Coleridge’s notes, shorthand reports of his lectures, newspaper reports and other things. More useful than Foakes’s full edition at first, perhaps, would be R.A. Foakes (ed.), Coleridge’s Criticism of Shakespeare. A Selection (1989) or Terence Hawkes (ed.), Coleridge on Shakespeare (Penguin, 1970); or look at the pages in Jonathan Bate (ed.), The Romantics on Shakespeare (Penguin, 1992) and the complementary pages in Joseph Wittreich (ed.), The Romantics on Milton (1970).
Much of the greatest prose appears in the Notebooks (edited in double volumes by Kathleen Coburn, 1957- ). There is a selected edition by me, published by Oxford University Press.
There is an extremely useful anthology of the great passages about Imagination in Coleridge, edited by John Spencer Hill (1978).
I can give you reading lists for more specialised tastes – Coleridge’s religious thought, philosophy and political commentary – as you want them.
(i) Accounts by Contemporaries.
Charles Lamb ‘Christ’s Hospital Five and Thirty Years Ago’ (London Magazine, 1830; in Lamb’s Essays of Elia, 1823).
William Hazlitt ‘Mr. Coleridge’ (in Hazlitt’s The Spirit of the Age (1825)); and ‘My First Acquaintance with Poets’ (The Liberal, 1823). Both these essays are collected - amongst many other places - in Hazlitt, Selected Writings (Penguin).
Thomas De Quincey ‘Samuel Taylor Coleridge’ (Tait’s Magazine, 1834-1835). Collected in Recollections of the Lakes and Lake Poets, ed. David Wright (Penguin).
Thomas Love Peacock Nightmare Abbey (1818). Collected with Crotchet Castle, ed.
Raymond Wright (Penguin). Coleridge appears as ‘Mr. Flosky’.
(ii) Some Introductions.
Leslie Stephen ‘Coleridge’; in Stephen, Hours in a Library (1879).
E.M. Forster ‘Silas Tomkyn Comberbache’; in Forster, Abinger Harvest (1934).
Virginia Woolf ‘Coleridge’ (1940); in Woolf, The Death of the Moth (1942).
Thomas McFarland ‘Coleridge’s Anxiety’; in John Beer (ed.), Coleridge’s Variety: Bicentenary Studies (1974); also in McFarland, Romanticism and the Forms of Ruin (1980).
M.H. Abrams ‘Coleridge and the Romantic Vision of the World’; in John Beer
(ed.), Coleridge’s Variety (1974).
John Beer ‘How Far Can we Trust Coleridge?’, The Wordsworth Circle 20 (1989).
Richard Holmes Coleridge. Early Visions (1989; part one of a biography).
Rosemary Ashton Samuel Taylor Coleridge: A Critical Biography (1996).
John Beer Coleridge the Visionary (1959).
Humphry House Coleridge. The Clark Lectures, 1953 (1962).
Walter Jackson Bate Coleridge (1969; ‘Masters of World Literature’ series).
Kathleen Coburn The Self-Conscious Imagination. The Riddell Memorial Lectures, 1973 (1974).
Ancient Mariner and the Supernatural Poems.
Robert Penn Warren, ‘A Poem of Pure Imagination’ (1946), in Selected Essays (1964).
John Beer ‘Poems of the Supernatural’; in R.L. Brett (ed.), S.T. Coleridge:
Writers and their Background (1971).
William Empson ‘Introduction’ to William Empson and David Pirie (eds.), Coleridge:
Selected Poetry (1972).
Jerome J. McGann The Ancient Mariner: the Meaning of the Meanings’; in The Beauty
of Inflexions. Literary Investigations in Historical Method and Theory (1985).
M.H. Abrams ‘Coleridge’s “A Light in Sound”: Science, Metascience, and Poetic
Imagination’ (1972); collected in Abrams, The Correspondent Breeze (1984).
Kelvin Everest Coleridge’s Secret Ministry. The Context of the Conversation Poems
Imagination and Critical Theory.
René Wellek ‘Coleridge’; in A History of Modern Criticism. The Romantic Age
R.L. Brett Fancy and Imagination (Critical Idiom series, 1970).
James Engell ‘Coleridge’; in The Creative Imagination: Enlightenment to
Jonathan Wordsworth ‘The Infinite I AM’; in Richard Gravil, Lucy Newlyn, Nicholas Roe
(eds.), Coleridge’s Imagination (1985)
Norman Fruman ‘Coleridge’s rejection of nature’; in Gravil et al, Coleridge’s
Partnership with Wordsworth.
Thomas McFarland ‘The Symbiosis of Wordsworth and Coleridge’; in his Romanticism
and the Forms of Ruin. Wordsworth, Coleridge, and the Modalities of Fragmentation (1980).
Lucy Newlyn Coleridge, Wordsworth, and the Language of Allusion (1986).
I.A. Richards Coleridge on Imagination (1934).
John Beer Coleridge the Visionary (1959).
H.W. Piper The Active Universe. Pantheism and the Concept of Imagination in
the English Romantic Poets (1962).
J.A. Appleyard Coleridge’s Philosophy of Literature (1965).
George Watson Coleridge the Poet (1966).
John Beer (ed.) Coleridge’s Variety (1974) - especially essays by George Whalley
(‘Coleridge’s Poetic Sensibility’), M.H. Abrams (‘Coleridge and the
Romantic Vision of the World’), Thomas McFarland (‘Coleridge’s
Anxiety’) and Owen Barfield (‘Coleridge’s Enjoyment of Words’).
Anthony John Harding Coleridge and the Idea of Love (1974).
John Beer Coleridge’s Poetic Intelligence (1979).
James McCusick Coleridge’s Philosophy of Language (1986).
Tim Fulford Coleridge’s Figurative Language (1991).
Seamus Perry Coleridge and the Uses of Division (1999).