There is a sense of privileged complicity in reading the letters of the famous. We are somehow present, hushed eavesdroppers peering over the shoulder of Henry James as he writes to his brother William, and there is a sweet, forbidden thrill in being so present – even if Henry is complaining, yet again, about his piles. Sometimes, too, we seem to attend the genesis of some rare insight, as when Keats, as if by afterthought, describes the experience of what he called “negative capability.”

Even better, though, are those letters through which we catch the passing moment, however homely. Then we seem almost to enter the scuffle and vexation of some lost instant, and we dwell in it, by the magic of immediacy. When Keats, en route to Rome and death, longs for Fanny Brawne and suddenly exclaims, in a letter of November 1, 1820, “Everything I have in my trunks that reminds me of her goes through me like a spear. The silk lining she put in my traveling cap scalds my head,” we are ourselves singed by that immediacy.

It’s hard to define what makes a private letter memorable. If forced, I’d say that it must contain the sudden pulse of the momentary. Madame de Sevigne’s letters to her daughter exemplify this to perfection. Like the keenest lens, her correspondence again and again excises some fragile sliver of time past and fixes it in seeming permanence. Yet her letters always feel impromptu, never premeditated. Some missives were consciously composed as literary artifacts, and I wouldn’t want to lose a word from the letters of Cicero or Lord Chesterfield or Samuel Johnson, but in these we admire the style, the attitude, the aplomb, as much as the personality overshadowing the words.

“The Letters of Robert Lowell,” now at last available in a sumptuous volume edited by Saskia Hamilton (Farrar, Straus & Giroux, 883 pages, $40), brims with immediacy. It is well known that Lowell suffered for most of his life from bipolar disorder, but it isn’t his bouts of mania that quicken the correspondence (though there are some distressing, even heartbreaking letters written in his “acute manic” phases). Rather, the warmth and force of his personality inform every line. Lowell must have been a taxing, even maddening, friend at times. Here, in letters dispatched to everyone from Ezra Pound and T.S. Eliot to Franklin D. Roosevelt, Lyndon Johnson, and Jackie Kennedy, some genial distillation of his essential self stands revealed. As I read them I couldn’t stop thinking: How wonderful it must have been to have received one of these vivid messages! And, such is the bustling intimacy of his letters, that for the first time I found myself, with steadily fading diffidence, calling him “Cal,” the nickname by which his closest friends knew him.

The letters of a great poet will always be held up against those earlier, quite incomparable exemplars, the letters of Keats and those of Byron. This isn’t fair, but would Lowell himself have wished for a lower standard? By this measure Lowell isn’t one of the great literary correspondents. Nor do his letters radically illumine his work, as those of Gerard Manley Hopkins or Rimbaud or Hart Crane do. Even so, Lowell was a great letter writer. His correspondence constitutes an incomparable account not solely of his own turbulent life but of the drastic period through which he, and many of the rest of us, lived (the letters begin in 1936 and end with his death by heart attack in 1977). I’m sure some readers will, as I did, find this book somewhat painful reading, for letter after letter, especially from the 1960s, summons up memories that are often bittersweet and sometimes excruciating.

Lowell’s letters must be taken as an aggregate. There are certainly wonderful individual letters, especially to Jean Stafford and Elizabeth Hardwick, former wives, as well as to such lifelong friends as Elizabeth Bishop and Peter and Eleanor Ross Taylor. Nevertheless, it is the cumulative and total effect that is most moving and most memorable. In this sense, taken as a whole, Lowell’s correspondence strikes me as the most important book to appear on the American literary scene in many years. It stands beside his “Collected Poems,” which appeared after endless delay in 2003, not simply as a complement but as a unique and indispensable creation in its own right.

Every page of “The Letters” is quotable (in this, Lowell does equal Keats), so the reviewer has an embarrassment of choice. In a 1964 letter to Bishop, to give but one example of Lowell’s austerely lyrical accents, he writes:

I went to sleep last night leaving the shade up over the open window at my ear because of the heat. What I looked at was the gorgeously rectilinear facade of an old-fashioned yellowish brick abandoned building. In the day, it is drab enough, and pigeons fly in and out of its broken windows, six to each of its five low stories. It seemed like South America, staid and vibrant sweep through the city. I woke with a half-dream vision of the utter metaphysical impossibility of our existence. Yet we had got by, we were, I was. And behind me stretched limitless spaces of glowing, pale, empty sky. It was a kind of peace.

You could say that there was a poem here tussling to be born, but the passage has what a Lowell poem, willed and obsessively revised, could not have: the precise impression of a reverie and, better, the progress of a reverie, from close observation to a very moving and unforced resolution: “It was a kind of peace.”

Other letters are far from peaceful. In January 1965, Lowell was treated for a manic episode at the Institute of Living in Hartford and described his surroundings in a letter to Elizabeth Hardwick:

During most of the day I transfer to a unit called Butler II, where the patients are adolescents, some wear long hair and almost none have finished high school. I won’t go into the boredom of “leather appreciation” and ceramics appreciation, of watching basketball games for an hour without smoking, or of trying to converse with the oldster reading Francis Bacon sentence by sentence and complaining but never specifying about the difficulties of the Elizabethan vocabulary, then turning to the boy, three months a freshman at some unknown university, [who] carry[s] around the Modern Library giant collected Keats and Shelley, but who had never heard of the “Ode to a Nightingale.”

The image of Robert Lowell enduring “ceramics appreciation” is painful to contemplate; yet it could be argued, on the evidence of many of the letters, that these forced sojourns in mental hospitals, despite the stupefying medications he had to take, afforded him some essential lull in his hectic life and perhaps even nourished his verse.

The volume includes letters on poetry, his own and that of others, to such coevals as Randall Jarrell, Theodore Roethke, John Berryman, as well as to Bishop, and these are quite down-to-earth and pleasingly specific. Lowell suffered many excesses, but he was never pretentious or self-important. His strong sense of his own worth as a poet enabled him to be, if not humble, then steadfastly modest. Even in dealing with his mentor Allen Tate, whose letters – to judge by Lowell’s responses – might have been penned in quills taken from a rampaging porcupine, all thrust and bristle, Lowell is invariably soothing and courteous.

The letters would be marvelous even if Lowell had never written a line of poetry, but they do provide the matrix out of which the great poems arose. They not only illustrate Lowell’s almost obsessive concern with public events – war, the civil rights movement, assassinations, the coarsening of our social lives – they also bring the man, in all the bigness of his blemishes as well as his great heartedness, grandly and poignantly before us. Tenderness runs through these pages alongside confusion, despair, and grief. After reading them I reread certain of his poems with a freshened eye, such as this excerpt from “Home After Three Months Away,” about his infant daughter:

Dimpled with exaltation,
my daughter holds her levee in the tub.
our noses rub,
each of us pats a stringy lock of hair –
they tell me nothing’s gone.
Though I am forty-one,
not forty now, the time I put away
was child’s play. After thirteen weeks
my child still dabs her cheeks
to start me shaving. When
we dress her in her sky-blue corduroy,
she changes to a boy,
and floats my shaving brush
and washcloth in the flush
…
Dearest, I cannot loiter here
in lather like a polar bear.

In his posthumously published “Book of Friends,” the Austrian poet and dramatist Hugo von Hofmannsthal wrote the following aphorism: “Only he who can create the tenderest can create the strongest.” That insight seems to me to capture the paradoxical quality of Robert Lowell in his poetry and prose and in these magical letters. The strength is evident in every poem and letter, sometimes indeed too much so. But in the best of them, it is strength made subtle by a clarity of tenderness.