HAROLD NICOLSON WAS BORN in 1886 in Tehran, where his father was British minister; he grew up in other exotic capitals, and in turn entered the diplomatic service. By the time he was 40 the smooth highroad to an ambassadorship lay before him. At that moment, this outwardly conventional man abandoned his cherished career for the perils of journalism and politics.

He was a natural writer: the paragraphs pattered effortlessly from his typewriter. His copious essays, reviews, radio talks, literary studies, biographies, and histories gave him a name, an income, and a reputation he deplored for "urbanity." But politics was a snare. Nicolson's convictions were social and personal, not political. Lacking an internal compass, he wavered from extreme to extreme, starting his progress with Oswald Mosley's New Party in 1930 (though extricating himself when its fascism became plain), ignominiously ending it by joining the victorious Labour Party in 1947 in hopes of a peerage. He died in 1968. He is remembered now for the one piece of writing he did without taking pains, the sparkling, perceptive diary he started as he began his new life, and for his unusual marriage, documented by his son Nigel in Portrait of a Marriage.

It was his commitment to his wife's happiness that made him give up a life that kept them apart; for V. Sackville-West (who refused to be called Mrs. Nicolson) rated her own writing too high to follow him about. Her husband never regretted the sacrifice; for though politics brought him many disappointments and rebuffs, and his writing seemed to him superficial and second--rate, his marriage remained a refuge, solace, and joy. It had a living metaphor in Sissinghurst, the Tudor house in whose scattered fragments they lived apart, and whose beautiful garden they created together. Both were homosexuals, she prodigal, he cautious, yet their letters, written while he was in London during the week, reveal tender, deep love on both sides. "If I believe in anything surviving," he wrote to her at the start of World War II, "I believe in a love like ours surviving -- it is so completely unmaterial in every way. My dearest, I felt so close to you yesterday. We never need to put it all in words."

The words, some 5 million, went into his diaries, a brilliant inside history of a critical decade. Three volumes of excerpts were published in the late '60s; the present book is a condensation of these. His view of government was from the back benches of Parliament from 1935, when he won a seat, until he lost it in the Labour landslide of 1945. His vivid account of the events leading up to the war and efforts to forstall, the fight and win it, is shaped by a sense of history and a novelist's delight in studying the actors with their pretensions, mixed motives, and obdurate quirks. He couldn't help seeing two sides to an issue -- bad for a politician, good for a diarist. The least fanatical of men, he is always alert, interested, sociable, recording conversations, shoptalk, and telling tidbits, small as a gesture like Churchill's running his hands up and down his tummy.

Churchill remained a hero to Nicolson long after others began to mutter, and he observed him closely in his frolics and depressions, often catching him in full oratorical spate, as when he tells Parliament unconditional surrender must not mean exterminating the Germans. "At this he takes off his glasses and swings round to the House, hitting his chest with his hand like an orang-outang. 'We remain bound,' he shouts, 'by our own customs and our own nature.'" Nicolson's Gibbonian eye seizes on the quiddities of De Gaulle, Lindbergh, Edward VIII, Lloyd George James Joyce, Nancy Astor, Hitler, Adalai Stevenson, Mrs. Simpson, Somerset Maugham, the Queen Mother, T. E. Lawrence, Alice Longworth. Yet he is himself as absorbing a character as any of these, more equivocal than most.

The strategy and daily challenge of the war that fill his pages fascinated him, yet he was sure that Hitler's destruction would not save the world he had known and loved. "All the delicacy of life will have gone," he lamented, adding later, "We imagine that we are fighting for liverty and our standards of civilization. But is it perfectly certain that by these phrases we do not mean the cultured life which we lead? I know that such life as lived by Vita and myself is 'good' in the philosphical sense. We are humane, charitable, just, and not vulgar. By God, we are not vulgar!" To this disclaimer he adds a characteristic rueful demurrer, omitted by the present editor: "Yet is it anything more than an elegant arabesque upon the corridors of history?"

All through the war he spent weekends at Sissinghurst, where Heinkels and Spitfires dueled overhead, weeding, writing, renewing his bruised spirits in Vita. Yet he had doubts about his writing as he did about his political life, wondering if he didn't use each to excuse failure in the other. "I feel this evening," he wrote while considering a three-volumeautobiography, "My inability present in front of me like a huge piece of furniture in a servent's bedroom." In 1944 someone showed him a review Edmund Wilson had written on his oeuvre, accusing him of being class-bound, inhibited, and lacking mental muscle. Nicolson saw the justice of this and was flattered that so distinguished a critic should take notice of him at all. Yet he kept at it, on one rainy weekend that foreclosed the garden churning out four articles, two talks, two reviews, and reading two books.

The Labour victory of 1945 ended his public life, in which he had become, a friend stammered, "a national figure -- of the s-s-s-second degree." Parliament had fulfilled his idea of perfect happiness: "genial surroundings and useful activity." Hoping to regain this paradise, he switched parties for the third time, taking socialism in a gingerly embrace. He failed to win a seat, nor did he secure the peerage he had hoped for. His fine biography of King George V later earned him a well-deserved knighthood, but of course he felt it was like getting a fourth prize in scripture instead of a first in history.

He is not a man likely to appeal to the 1980s. He did not love the masses; he as sentimental, sometimes silly, second-rate, maybe, by his own high standards. But what fine old-fashioned virtues he had: He worked very hard; he valued civility and practiced it; he was witty, companionable, and utterly loyal firend, a loving husband and father. And he was happy. He feared this was a sign of being superficial, too fluid: "But few people have extracted so much pleasure from fluidity, and when I look back on my life it is as gay as an Alpine meadow."

These abruptly condensed diary entires do him little justice. The dailiness of life through a momentous epoch is half the point, and it's scamped; his lucid style, with its color, detail, and quirks of phrasing, is clouded by unacknowledged paraphrase and precis. We miss the whole man by not knowing that after a reasoned analysis of his marriage, he exclaims: "Oh, my precious, my gentle, my magnificent Viti!"