HOW RARELY is a life as cleanly divided down the middle as Harold Nicolson's was. The second volume of James Lees-Milne's fine biography of this man of many parts begins at the traumatic moment when he quit the world of diplomacy and made a new beginning as a writer and politician. In most lives the second half rarely comes up to the first, with its fund of childhood memory, but the interest of Harold's veiled personality, the people he knew and events he witnessed, his marriage -- a mysteriously successful union between a homosexual man and woman -- and the web of mistakes, misfortunes, and happy deliverances that marked his life, make the second volume a match for the first.

Son of Britain's ambassador to Persia at the time of his birth in 1886, Harold Nicolson was destined for a diplomatic career and showed extraordinary talent for it. However, Vita Sackville-West, whom he married in 1913, put her career as a writer of commonplace novels and prosing poems above being Mrs. Nicolson, and though her husband's goal was an embassy, she would not be an ambassador's wife. He could have achieved the highest posts -- Paris, Washington, Berlin; he loved everything about diplomacy and had made invaluable contributions during the First World War and at the peace conference in 1919. Had his lucid and exact memoranda about how to deal with Germany been heeded, the 20th century might have been spared the legacy of Versailles.

Egged on by the infatuated Virginia Woolf, Vita pushed Harold to the edge. If he gave up his career, he told her, "merely for emotional reasons, I should feel a worm -- unworthy of what is one of the few serious and virile sides of my nature." Forced to choose between wife and career, in 1930 Harold turned his back on a world in which he had been masterfully at home to write a newspaper gossip column and give BBC talks. His matter, both frivolous and serious, and his manner -- "cosy, hesitant, but authoritative" -- made him a great success. To him it rang hollow. "I want to be in real things again," he told Vita. Eager to use his intelligence and experience at a time of national crisis, he joined the proto- fascist Oswald Mosley's New Party, attracted not by the black shirts, which were still to come, but by its Keynesian economic solutions to unemployment. "He was all his life an intensely honorable man," Lees-Milne writes, "incapable of personal advantage at the expense of his principles. His honesty was transparent, and often naive." Only when it was no longer possible to doubt where Mosley was headed did Harold leave him. "Everything has gone wrong," he wrote at the end of 1931. "I have lost not only my fortune but much of my reputation." A natural ebullience that never failed him until the end made him add, "Yet in spite of this, what fun life is!"

HE HAD many sources of delight: two sons, who regarded him as "the nicest father," an adored wife, home. In 1930 the Nicolsons bought Sissinghurst, the ruinous fragments of a Tudor castle, where they would henceforth live -- or, perhaps more accurately, camp out, each in his separate bit -- and where, between them, Harold its classically-minded architect, Vita the romantically lavish plantswoman, they created perhaps the greatest garden of the age. It was to provide them with profound joy until they were old and ill and beyond caring.

No less important to Harold was his literary career, and in this Vita took a keen interest, rating him higher than he too modestly rated himself. He always worked very fast and methodically, beginning each book on the first of the month. Two big books completed within six months was nothing exceptional, on top of which he carried a heavy load of book reviewing, regular essays and columns, broadcasts, and speeches. The variety of the many books he wrote suggests the breadth of his interest: fine biographies of the poets Byron, Tennyson, Swinburne, and Verlaine; shrewd and subtle diplomatic studies and histories; collections of essays; novels (one, in 1934, predicting the atom bomb). After the second war the royal family asked him to write a biography of King George V. After vast research, he faced the writing: "like starting in a taxi to Vladivostock." Queen Mary loved it; so did the public. It made him the first real money he had ever had and he gaily spent it entertaining his friends. But because it would not have occurred to him to move to a tax haven, the Inland Revenue took most of it away.

He went on to write admirable biographies of Benjamin Constant and Sainte-Beuve. "His personality peeped through whomsoever he was dissecting," Lees-Milne writes. "He saw all men as near-failures and their thwarted ambitions as poignant." In a series of essays titled Good Behavior he expressed his views of life as plainly as he was ever to do. The unobtrusive elegance of his supple prose was admired even by detractors like Edmund Wilson, who thought him a snob and a prig.

Yet he never became the writer he wanted to be and might have been had only he been able to open himself without reservation. Early in the 1930s he conceived of a magnum opus, a Proustian autobiography in some 10 volumes, comprising not the half-truths in which Proust dealt, but the whole truth. Alas, he who would not speak of sex even in his diaries, and who had written about Verlaine never mentioning his relations with Rimbaud, faltered at this prospect. As he saw it, it was the fear of facing the challenge of this unwritten book that diverted him into politics. When a lesser book containing childhood recollections was described as "Proustian," young Evelyn Waugh exploded: Proust's work was his life;Nicolson was a man of many interests who wrote as recreation. In spite of his "almost effortless grace of expression and humor," Waugh considered him in effect a dabbler. Vita rightly observed that whatever shortcomings he had as a writer were to be blamed on his "morbid dislike of emotion."

As he bemusedly remarked shortly before his death, what he is best remembered for is something he never knew he had written, his letters and the diaries begun in 1930, selected and edited by his son Nigel. Here at least the tender depth of his feeling for his wife is poignantly expressed.

POLITICS, after a false start, was to provide him with some of his happiest years. From 1934 until 1946 he enjoyed membership in that great club the House of Commons, where he was respected for his specialized knowledge and his transparent integrity. But he had more desire to please than to dominate and in his wish to be liked kept failing to exercise self-interest. Yet if he lacked toughness and ambition, he was no cowad, and took lonely, unpopular positions when his conscience bade him.

When war came, he knew that, no matter who won, it was the end of all he cared for, but he faced it with patriotic fervor. At the outset he enjoyed a minor post in the Ministry of Information, but his idol Churchill soon callously turned him out. Deeply hurt, he saw that he had no political future. But his loyalty to Churchill was unscarred by his disappointment and he defended him even over the Yalta agreement.

The Labor victory of 1946 that washed Churchill out of office eliminated Harold too. In hopes of regaining a seat, he joined the Labor Party. His sons were dismayed, Vita appalled, his mother outraged, while his brother "tartly observed that he supposed Harold would resign from all his clubs." Only Eden defended him: "He is a person apart and has every right to his own decisions. He will do a world of good in the Labor Party."

Whatever good he might have done, he was trounced, and to make matters worse devoted an irreverent column to his ordeal: "A good candidate should be convinced that he is more intelligent, far more honorable, and infinitely more valuable to his country than any of his opponents. I have never been adept at that sort of thing." The impertinent column played a part in his failure to garner the title he desired. He belonged to a class that set no store by new titles, but regarded elevation to the peerage as the natural reward for a lifetime of service. Never doubting, when he naively asked for a title, that he would receive it, he joked about a suitable one: Lord Sissinghurst wouldn't do, of course; his friends would call him Lord Pansyhurst. Far more important to him than the handle, a peerage would have given him years of useful service in the House of Lords. He minded terribly when he realized he would never be Lord Anybody.

AN UNWELCOME knighthood was foisted on him, a shabby substitute for what he deserved. It made him look foolish, he thought, "like someone who, having expected to be put in the first eleven, found himself in the third." He was deluged with honorary degrees and marks of distinction; he was a prized lecturer and guest everywhere. But when the Poetry Chair at Oxford came vacant, though he put his name forward he refused, all too typically, to campaign for the post. Instead, by a few votes, it went to Wystan Auden. Had Harold been willing to say what he thought -- that a man who had deserted England when war threatened ought not to be so honored -- he might have prevailed.

He was well beloved by many men of all ages and was in turn the most loyal and helpful of friends, writing prefaces for their books, lending them money -- and prompting his secretary to remark that it would be "a great economy when all your friends are in prison." One of them who would indeed have gone to prison had he returned from Moscow was Guy Burgess. In spite of deploring the damage his defection had done the Foreign Office, Harold correspondedwith this pariah for years. "Nothing a person, who had once been a friend, could do, however offensive, erased in Harold's sentimental mind the memory of that friend when he had first known and been attracted to him."

He had traveled widely and continually all his life; now he and Vita began taking winter cruises, tucked up in separate staterooms writing their separate books, yet regarding these adventures as a second honeymoon. "Both of them had long been obsessed by the unorthodoxy of their marriage, and convinced that this unorthodoxy, coupled with the tolerance and understanding of both, was the main reason why their marriage had not only endured, but prospered," Lees-Milne writes, adding that had one been homosexual and the other not, things might have been different. Vita's death in 1962 knocked the stuffing out of Harold. He went on living at Sissinghurst with his son's family, idle at last, tearful, sometimes uttering the self-pitying moan, "Povere vecchio!"

At his 75th birthday party, before Vita died, he had been asked if he had any regrets. He was sorry, he admitted, that he had been bad at games, for it ruined his schooldays, but he did not regret never having been an ambassador. He wished he knew science; poetry was the most civilizing of the arts; the purpose of life was happiness -- happiness in the sense of being useful and developing the best in oneself. He had been useful and had worked very hard at being his best self, and yet a sense of what if lingers. Never to have been an ambassador he came not to mind; that he had not used his extraordinary gifts and knowledge of many worlds to embark on an English search for time past is what we must regret, as surely he did.

James Lees-Milne came into Harold's life early in the 1930s as one of his son's friends and stayed on to the end as one of his own nearest and dearest. Now, late in his own career, he has pulled off an extraordinarily fine biography, vividly alive, beautifully balanced between the public and private spheres of Harold's life. It compares in fascination with Harold's irresistible Letters and Diaries.