THE FOUR COLLECTIONS reviewed here -- all solid, and one quite magnificent -- vary widely in style and suasion and temperament. It's unlikely that the same reader will be able to respond with equal fervor to, say, Alfred Corn (all subtlety and finesse) and Paul West (ardor bordering on good-natured bluster). What these critics have in common is simply their old-fashioned bias for real books over postulated texts. That, and their shared notion of the critic's responsibility. Monroe K. Spears, in an essay on Cleanth Brooks, spells it out: the critic shall be responsible "to his readers or other audience; to his authors, whether living or dead; and to whatever standards of truth he believes in."

Robert Lowell had doubts about his critical abilities. "I am not myself a practiced critic," he wrote in an appreciation of Yvor Winters, "and have no gift for the authoritative and lucid comment that somehow makes a quotation sail." Lowell's Collected Prose (Farrar, Straus and Giroux, $25) belies such misgivings. It is, judged by the strictest of standards, an extraordinary body of work. Part Three of the book, which contains the poet's critical self-appraisals and autobiographical prose, alone makes it indispensable for any serious reader of Lowell -- which means any serious reader of modern poetry.

Lowell's self-effacement aside, he transformed himself into a superb critic by learning to lodge his trust in his immediate and subjective responses. By the 1960s -- the decade in which the parallel lines of the poet's biography and the nation's history kept intersecting -- he had wisely decided to ditch "the standard analytical essay" in favor of something "much sloppier and more intuitive," as he told his Paris Review interviewer. His own poetry, once so metrically gnarled, was now being written to a prose beat (Life Studies, 1959). And his prose had become an instrument of rugged eloquence -- unshackled by convention, personal without being private, wholly idiosyncratic and wholly authentic.

A handful of quotations from the Collected Prose will suggest the authoritative and lucid character of the prose style Lowell invented for himself. On the "Dantesque" nature of W.C. Williams' American journey: "His flowers rustle by the superhighways and pick up all our voices." On the art of Sylvia Plath, so drastically different from the "savage" ideal of the poe`te maudit: "This poetry and life are not a career; they tell that life, even when disciplined, is simply not worth it."

No one will ever characterize Lowell's style as effortless. Reading this book one is consciously aware of the writer's struggle to bend the syntax to his will, his sense of the medium's resistance, his urgent need to burst out of his own rhetorical confines. The effect is one of intense compression: insight lifted to aphorism, description sharpened into portraiture. A string of alliterative adjectives combines with a surprising simile to produce this stunning snapshot of Ford Madox Ford: "Ford was large, unwieldy, wheezy, unwell, and looked somehow like a British version of the Republican elephant."

The chain of adjectives became a favorite Lowell device, as though it were the nearest thing in prose to a metrical standard: the effect of blank verse without the necessity of the iambic pentameter line. So, for example, the young Jarrell was "unsettlingly brilliant, precocious, knowing, naive, and vexing" while Lowell himself, at the Payne-Whitney Clinic following one of his breakdowns, was "distant, thorny, horny, absentminded, ineptly polite, vacantly rude." Subtract any of those adjectives, and see how quickly the effect is ruined.

If Jarrell was the great celebrator of a generation's juvenescence, it was Lowell's destiny to be its elegiac scribe. His eulogies to the fallen poets of his generation (Jarrell, Berryman, Plath) and to the old masters (Eliot, Williams, Frost) have an air of almost unbearable sadness about them. These critical retrospectives seem to fall into a genre of their own: acts of farewell, repeated ritually, in which anecdote and analysis are woven so closely together that the art and the life of the mourned-for author seem somehow to coincide.

Robert Giroux has done as deft an editorial job on Lowell's Collected Prose as he previously did on posthumous collections by Berryman and Elizabeth Bishop. Among Giroux's "discoveries" perhaps the most remarkable are two that were never completed: "New England and Further," Lowell's statement of the American sublime, and "Art and Evil," which is haunting and fascinating as only a well-wrought fragment can be. The latter breaks off eerily in the middle of a joke ("a popular moralist's practical joke, the joke of the torturer boiled . . . ") intended to illuminate the condition and fate of Iago; we never arrive at the punchline.

The Collected Prose concludes with Lowell on Lowell: his Paris Review interview, his comments on "Skunk Hour," a 1971 conversation with his future biographer. "91 Revere Street" is reprinted alongside two previously unpublished companion pieces quarried from the same autobiographical project: "Antebellum Boston" and the harrowing "Near the Unbalanced Aquarium."

Collections of critical essays face a common problem. What determines their unity? What turns a miscellany of reviews, written under the pressure of deadlines and publication dates, into a volume of essays worthy of our interest years after the original occasion? "This volume is obviously not an assemblage and was not conceived as a single book," Alfred Corn writes in his preface to The Metamorphoses of Metaphor (Elisabeth Sifton/Viking, $18.95). Corn argues that his book "has roughly the same kind of unity found in a collection of short stories." NEVERTHELESS, The Metamorphoses of Metaphor does add up to more than the sum of its piecemeal parts -- in large part because Corn, an accomplished poet, illustrates in his prose style the theme that his title adumbrates: "Metaphors metamorphose under our very eyes. They may serve to shed the light of meaning on some small homely object, as when Elizabeth Bishop compares the herring scales stuck to an old fisherman's shirt to sequins; or suggest an overarching structural analogy, as when Proust makes us see his long novel as a cathedral, one that is also sailing down the river of Time like the ark or the ship of Faith." What Corn offers is not so much a thesis as a lesson in sensibility and in the reflective power of the imagination. Metaphors, he demonstrates, are images that yet fresh images beget.

Corn's reflections on recent and contemporary American poets are particularly resonant. He observes that for Wallace Stevens, the maestro of metaphor, the "transformation of reality" amounted to "a religious pilgrimage": the quest for a supreme fiction. Very different from the life of metaphor led by Lowell, determined as he was to write "history's autobiography." Both are instances of the American tradition to "treat fact allegorically" and a less sensitive critic might push the point. For Corn, it's almost an afterthought -- his emphasis is, as it should be, not on the tradition but on the individual talents who extend and sometimes subvert that tradition. His interpretations of poems by Lowell ("Rebellion," "Beyond the Alps") and by Elizabeth Bishop ("Seascape," "The Map") are shrewd, persuasive, succinct.

At his best, Corn relies on his own metaphorical dexterity to render his subjects' motive for metaphor, as when he approaches John Ashbery's poetry from this deliciously vertiginous angle: "Anyone who has seen one of the old cartoons in which cat, coyote, or luftmensch walks over the edge of a cliff and navigates successfully until he perceives it was by blind faith alone knows that laughter may be metaphysical -- and this scenario is one that occurs, varied and abstracted, in many of Ashbery's poems."

In Sheer Fiction (McPherson & Company, $17.95), Paul West votes for verbal gigantism, a high-caloric linguistic pleasure principle. West, whose 10 published novels include Rat Man of Paris and The Very Rich Hours of Count von Stauffenberg, is an unabashed proponent and practitioner of purple prose. He is for pageantry, against austerity; for prose that is "revved up, ample, intense, incandescent, or flamboyant"; against any sensibility that would regard "taut, clean, crisp, tight, terse, lean" as virtues; for baroque elaboration and sheer invention, against naturalism in any narrow sense. The foremost enemy is trendy "minimalism," a dandy punching bag. Minimalism, West writes, is "the ponderous ho-hum of the gull who thinks fiction somehow photographs life instead of mimicking life's creative ways. Minimalism to me is what there cannot be too little of."

The essays in Sheer Fiction rely on digressions, deviations, and bravura displays of associative logic to enact their themes. Take, for example, "A Rocking Horse on Mars" (one of several marvelous titles), which begins as an autobiographical memoir ("I was born in Lady Chatterley's village"). When we arrive at the author's discovery of William Faulkner ("Sometimes there were four pages of narrative to one word of dialogue -- an act of sublime defiance"), we seem to be in familiar Wordsworthian territory, charting the growth of the writer's mind. Suddenly, the essay makes a sharp right turn -- or, rather, it signals right and proceeds to turn left. Reminiscence breaks off; what we get instead are scientific wonders and metaphysical speculations. By the close of the piece, the argument (for there remains the structure of an argument, even if it eludes paraphrase) has transformed itself not once but several times, with the freedom of a poetic equation, the freedom of "sensuous opportunism."

"Sometimes," West writes, "the game we play with literature interests us more than literature itself." This is often the case with West himself. There is something compulsive in his procedures, as if he cannot ask a rhetorical question without needing to exfoliate it into five others. The result occasionally resembles a feast of words that go down smoothly but leave us feeling a little hungry again a half hour later. There is also the tendency to overstate the case, as when West asserts that "sheer consumerism has always kept the novel back, making it into a commodity rather than an art form." Didn't Dickens prove it was possible for the novel to be both?

It's impossible, however, to stay mad for very long with anyone who writes so well, and with such evident zest, about a time when minimalism will have breathed its last, when novelists will feel as "technically" free as abstract expressionists, and when "literary art may never offer a beginning, a middle, and an end, in that order, again, except out of nostalgia." An extreme statement? To be sure, but sometimes those are the most effective kind.

The essays in American Ambitions (Johns Hopkins, $24.50) display the prose virtues of the better literary quarterlies -- the ones not yet decimated by deconstructionists, that is. Best known for his excellent critical study of W.H. Auden, Monroe K. Spears edited Sewanee Review for nine years during the age of criticism. His central concerns are poetry ("Life Function of Literary Quarterlies," fine essays on Tate, Blackmur, Brooks, and Warren). But he can also branch pretty far afield, from Cotton Mather to "Jewish Intellectuals of New York." His writing is serious, conscientious, exact, plain. No fancy verbal fireworks coax the reader into paying attention; it's expected that you care.

The most ambitious of these essays -- the one that gives the book its coloration -- is "Revolution in American Poetry," which ponders the relation of political and literary revolutions. Our declaration of literary independence did not occur, Spears argues, until nearly four score years after the American Revolution. But if, as he notes, "the American Revolution has thus not been very important as a specific subject in American poetry," it has been an omnipresent theme.

Our writers have translated the revolutionary mandate into a formal challenge: "In literary terms this tends to mean breaking with the conventions of literary form and celebrating the self free, untrammeled, and innocent." The argument is made with exemplary clarity -- another of Spears' prose virtues. :: David Lehman's most recent books include a collection of poetry, "An Alternative to Speech" and "Ecstatic Occasions, Expedient Forms," in which 65 contemporary poets comment on form in favorite poems.