"WE DANCE ROUND in a ring and suppose,/But the Secret sits in the middle and knows." The dancers in Robert Frost's couplet could as well be taken for those who have sought to solve the riddle of this titanic, contradictory, vexing, sometimes petty, and often great-souled poet. One sterling critical book -- Robert Frost: The Work of Knowing , by Richard Poirer -- and a passel of academic and sentimental speculations and memoirs make up the literature investigating Frost's life and work, along with the unworkably iterative three-volume biography by Frost's own elected portraitist, Lawrance Thompson.

Perhaps the Secret is, as Santayana said, that no good deed goes unpunished. No other friend of Frost's was more wholly or unselfishly devoted to Frost's work during his lifetime than Sidney Cox. Newly graduated from Bates College, Cox was 22 when, in 1911, he and Frost met while teaching at separate schools in Plymouth, New Hampshire. Frost himself was 37, had yet to publish his first book (A Boy's Will ) or to write his second (North of Boston ). The older man, after some 30 years as a planetoid of Lawrence, Massachusetts, and the satellite towns near it in Massachusetts and New Hampshire (Methuen, Amesbury, Salem, Windham, Derry), was now nearly ready to undertake, as he wrote to a receptive editor at the time, a "long deferred forward movement." A smaller leap -- the hundred miles from Lawrence to Plymouth -- he had already ventured in order to teach girls at the New Hampshire State Normal School, far from home at last.

Avid young Sidney Cox, once he got accustomed to Frost's captiousness and pawky humor, came to adore his mentor. He relished his talk, joyfully accompanied him on walks through woods and mountain intervals, exulted in his friendship. Widely, even deeply educated, Cox carried an intensity of enthusiasm which appealed to Frost, though now as later in life Frost would have some difficulty with his friendships, friendships which in the fullness of time would amount to hundreds, thousands. The most ingratiating of talkers, with his wickedly twinkling humor and a disarming confidentiality, Frost struck Cox as "the most perfectly sincere man I know." Yet the pattern of advance and withdrawal which Frost established with Cox would be often repeated.

Cox admired and pursued. Frost assented, evaded, resumed, embraced, escaped. Cox and Frost, once Frost had found in Plymouth the poetic voice he went to England to train in 1912-1915, corresponded with eager excahanges between Poland, New York and Little Iddens, Gloucestershire, with Frost employing Cox as U.S.A. publicist and even as catspaw: "You have been splendid. Poetry needs just the kind of help you are giving me."

When Frost returned to America he was teetering on the edge of fame, and thus many appreciative witnesses crowded in: Amy Lowell, Louis Untermeyer, Joseph Warren Beach, William Stanley Braithwaite, Gorham Munson. Cox, like a dozen others, wanted to proclaim the marvels of Frost to the world, but the moment each witness stepped forward, Frost sashayed out of sight. Cox, in 1929, published Robert Frost: Original "Ordinary Man ," a perceptive little appreciation of Frost which the poet either didn't read, or pretended not to. The fat was sizzling in the fire. A hint of estrangement, long prepared for, now crept into this relationship. Frost felt crowded. The man who had told Cox, "You set me off," and in 1914, "I write few such long letters to anyone as I write to you," arrived eventually in 1932 at near-insult with such a remark as, "I have written to keep the over curious out of the secret places of my mind both in my verse and in my letters to such as you."

Later years brought alternating reconciliation and estrangement. Cox faithfully taught Frost's poetry along with Shakespeare's and Wordsworth's to a procession of gifted and distinguished students at Dartmouth College -- where, ultimately, Frost himself came to spend part of each year. Over decades Frost was often in Cox's company until Cox, burned out by the dedication and intensity of 12-months-a-year teaching, underwent several heart attacks and died at 62 in 1952, 11 years before Frost. Nothing in William R. Evans' book is more touching or revealing of Frost's weakness and the ambiguity of his literary friendships that the posthumous correspondence in which Cox's publisher, Wilson Follett, entreated Frost to write an introduction to Cox's life work, an appreciative book called A Swinger of Birches . Their correspondence displays the Frost fox trot at its most syncopated.

Of the 134 letters included between Cox and Frost and their families, 38 of Frost's letters, and most of the best, were published in 1964 in Lawrence Thompson's Selected Letters of Robert Frost . These recount Frost's early attitude toward his own prosody, his early breakout in 1912-13 into the "New England eclogues" of North of Boston and Mountain Interval . "You have no idea of the way I mismanage miself since I broke loose and ceased to keep hours. It seems as if I did nothing but write and write." "I say you cant read a single good sentence with the salt in it unless you have previously heard it spoken. . . .Words exist in the mouth not in books." "Perhaps you think I am joking. I am never so serios as when I am." "Look out I don't spoof you."

In addition to the superb Selected Letters , four important sequences of Frost correspondence are now extant: Letters of Robert Frost to Louis Ntermeyer , (1963) Robert Frost and John Bartlett (1963) Family Letters , (1972), and now this comparable, well-edited book. Cox was at once the most keen and the least wordly of Frost's close correspondents and perhaps the one most vulnerable to injury. This exchange is both fascinating and melancholy, matching a heavyweight with a Golden Glove fighter in a match that only one of them recognized as a contest. The heavyweight showed his class throughout, a quality of form that only champions reach. "To be too subjective with what an artist has rendered objective is to come on him presumptuously and render ungraceful what he inpain of his life had faith he had made graceful." Frost felt he had a right to be read well, and in such statements the reader can forgive the foxiness of the trot because of the gravity of the music. Frost's concern for lthe harmony that art could bring into the discordancies of life overmastered everything else in him, even friendship.