THE FRINGES OF POWER: 10 Downing Street Diaries 1939-1955. By John Colville Norton. 796 pp. $25. AT THE HEIGHT of the Battle of Copenhagen, Lord Nelson, who had lost his right eye in an earlier action, received a signal from a timid British admiral ordering him to disengage. He ignored it, telling his officers: "I have only one eye -- I have a right to be blind sometimes." Because he went on to forge a magnificent victory, "showing Nelson's eye" has become synonymous with splendid insubordination, though seldom more engrossingly than in these diaries of Sir John Colville. World War II was barely a month old on October 3, 1939, when "Jock" Colville, then a 24- year-old civil servant in the Foreign Office, was appointed private secretary to the prime minister at No. 10 Downing Street. Neville Chamberlain was in residence at the time, but seven months later the House of Commons, wrenching power from Chamberlain, turned to Winston Churchill, and this book -- over 81 percent of it -- is largely Churchill's story. Except for brief periods (including D-Day) when Jock braved the P.M.'s wrath and flew as an RAF fighter pilot, he was almost constantly at Winston's side.

Written with precision and clarity, The Fringes of Power is a treasure for scholars and, for the general reader, a compelling narrative. A wartime rule forbade the keeping of written personal records, but Colville was a compulsive diarist. So he kept the accumulating volumes of his diary in a safe place at No. 10. Their eventual length is unrevealed to us; this is an abridged version of the original, now under seal at Churchill College, Cambridge. But they must be vast. Of February 9, 1941, he writes: "At this stage of the proceedings I decided to leave the three fat volumes of my diary locked up . . . because its indiscretions were considerable." But he had scarcely turned the key before he started a small pocket diary which grew and swelled and multiplied.

He had been warned that this was "dangerous." When the P.M. circulated a memorandum about preserving the secrecy of documents, Colville noted that it suddenly made him "feel rather conscience-stricken about this diary. I haven't the heart to destroy it and shall compromise by keeping it locked up here, even more strictly than hitherto." We can only be grateful for his audacity. The wages of his sin are, for us, a priceless legacy, particularly his day-by-day account of Winston's activities during the year after Dunkirk, when he and his countrymen, armed only with righteousness and wrath, held their island free from Hitler's bloody grasp.

If the book has a flaw, it lies in the title. The diarist was no fringe spectator. He was in the cockpit of action, a trustee of delicate confidences, and as time wore on he became a man of sound judgment whose suggestions were weighed and often adopted. One explanation for his rise lies in Jock's intellect. At Trinity College, Cambridge he had been awarded first-class honors in history, and his subsequent performance in the Middle East as a junior diplomat had been brilliant. He found diversion in Milton, Keats, Shelley, Byron, Lord Chesterfield, and Lord Rosebery's essays and like Churchill he enjoyed Trollope. (Neither qualified as an astute critic of serious films, however. Orson Welles' Citizen Kane, now regarded as the most innovative and perhaps the greatest motion picture of all time seemed "deplorable" to Jock, and Winston walked out on it.)

But there was another reason for his emergence. In the 1930s the privileged class still dominated His Majesty's Governments; of the 29 men who dominated policy, 23 were products of elite "public" (boarding) schools -- 17 of them Etonians -- and all but seven were Oxbridge men. Jock met every patrician standard. He was a Harrovian, the cousin of a viscount; he opened each sunlit morning with a brisk gallop through Richmond Park and lunched at the Travellers, White's or the Turf; his grandfather had been a friend of Churchill's father; he was a frequent guest of Queen Mary, whose close friend and lady-in- waiting was Jock's mother.

FOR ANY DIARIST who hopes that his work will live, discretion lights the way to dusty death. Like Boswell, Jock never hesitated to set down rumors, gossip, his own prejudices, and his judgment of others, which he later found unreliable. He described Harold Macmillan as "finicky and probably a little insincere." Duncan Sandys was "an opportunist." This volume is dedicated to Lady Soames, Winston's daughter Mary, but when he first met her she struck him as "rather supercilious." Of Mary's mother, the formidable Clementine, he noted: "We talked a good deal of politics, about which her views are as ill-judged as they are decisive," and after another encounter with her he wrote: "Mrs Churchill was abusive. She seemed to expect me to make the arrangements for a visit to Glasgow on Monday. . ." Jock was busy and objected to "acting as Mrs C.'s secretary"; therefore he passed her instructions on to Grace Hamlin, Clementine's secretary. Clemmie was "furious," he wrote in that day's diary entry, "and said I gave myself airs, etc., etc. . . . Mrs C. considers it one of her missions in life to put people in their place and prides herself on being outspoken."

Colville had been an enthusiastic supporter of appeasement (later Churchill would taunt him as a "Munichist") and even after the outbreak of war he had deplored a government white paper on German concentration camps, calling it a "sordid document calculated to appeal to people's lowest instincts. . . . after all, most of the evidence is produced from prejudiced sources, and it is in any case undsirable to arouse passions."

During the "Bore War" of the winter of 1939-40 he acknowledged that Churchill was "the only man in the country who commands anything like universal respect" and would probably become prime minister before the war's end, but added that "judging from his untrustworthiness and instability, he may, in that case, lead us into the most dangerous paths." When King George VI (who shared his doubts about Winston) handed Churchill the seals of office, Jock quoted R.A. Butler as saying the prime ministry had been "sold to the greatest adventurer in modern political history. . . . a half-breed American."

However, once the new prime minister had rallied the dispirited, defeatest country, a universal joint shifted somewhere in Britain's national mood, and Winston's conquest of Colville's heart began. Americans, who knew little of Churchill until he began running the crusade against Hitler (running it from an office in the Admiralty; it took Chamberlain a month to move out of Downing Street) cannot understand why Englishmen were slow to grasp his greatness. As we watch him through the eyes of his private secretary, however, we see how his public's perception of him changed. Only 18 months earlier they had rejoiced in the shabby deal at Munich. But in 1940, as Colville's admiration of him grew, the nation's grew also. Rarely in history has a leader actually led his countrymen from the garden path to the paths of righteousness, transforming them from sheep to lions. Nine months after Winston became prime minister the diarist wrote that "it is an interesting spotlight on No. 10 last winter that he should have been regarded with such dislike and distrust." By then Jock had been all but adopted by the Churchill family. He frequently dined en famille with "the P.M., Mrs C. and Mary," and was privy to family secrets and familial celebrations. This book, therefore, is very much the work of an insider. The general reader could easily get lost in so large a cast of unfamiliar characters, but Jock, anticipating the problem, identifies 117 major figures with asterisks in the text; these lead one to encapsulated biographies, some of them gems, at the end of the volume.

IN THE HANDS of gifted men, diaries may approach the level of literature. Perhaps nothing written about the Fall of France in 1940 is so gripping as Colville's entries between May 10, when Churchill became prime minister even as the German Wehrmacht knifed through Belgium and Holland into France, and May 31, when Jock wrote of Dunkirk: "One of the world's greatest defeats is being redeemed by an outstanding achievement of . . . gallantry." The accounts he set down during those three weeks are often inaccurate. Frequently they omit key developments. But that was all that anyone in England knew at the time. And it is precisely this which gives them their extraordinary sense of immediacy. Turning the pages we hold our breath, feeling that it is all happening for the first time even though we know, as the diarist could not, that the Nazi threat to invade Britain would founder.

We are familiar with the broad outlines of the tale and often of details. What Colville provides is an entirely convincing portrait of Churchill in the flesh, an incomparable fighter, generous to the conquered but inconsiderate of his staff, a user of men who discarded them without a thought, incapable of apologizing though charming those he had hurt, ruthless -- even cruel -- yet a statesman of soaring vision, possessed of that single-mindedness which William James defined as the essence of genius, of an eloquence which held the entire House of Commons under his spell, of a wit that disarmed his opponents, and of a pixie quality which his overworked staff found irresistible. On May 1, 1940, when Colville first met Churchill -- then First Lord of the Admiralty under Chamberlain -- a steady rain was drenching the Horse Guards Parade outside No. 10. Only Winston, staring out bleakly, could have muttered: "If I were the first of May, I should be ashamed of myself." He was le pass,e maitre of the unexpected remark; five years to the day after that downpour he learned at dinner that Adolf Hitler had poisoned his dog, murdered his wife, and killed himself, and that on his instructions his corpse, soaked in gasoline, lay in his ruined Reichskanzei garden, enveloped in writhing flames. Winston reflected a moment and remarked: "Well, I must say he was perfectly right to die like that."

He never concealed his aristocratic stigmata. Nevertheless his support in other classes was solid; shopkeepers and working men were delighted by his deliberate mispronunciation of foreign names (not only "Nahrzees," but also "Marsales," "Lie-ons," and "Calase,") though sometimes this affectation offended Gallic sensibilities. Scheduled to broadcast an appeal to the French in their own language, he entered the radio studio, where a BBC delegation, including the network's French expert, M. Duschesne, awaited him, Looking around Winston asked: "Where is my frog speech?" Colville notes: "M. Duschesne looked pained."

Like Hitler, Churchill had his underground bunker, an air-raid shelter built beneath Storey's Gate, a short walk from No. 10. On a typical evening during the Blitz, the staff raced down the stairs as sirens wailed while the P.M., wearing a flat British steel helmet and what Colville described as a "magnificent golden dragon dressing gown" headed for the roof to count the Dorniers and Heinkels caught in the searchlight beams overhead.

Throughout the war he went about heavily armed. Colville describes how, puffing on a cigar, he fired his revolver and Mannlicher rifle at ranges up to 300 yards and, "despite his age, size, and lack of practice, he acquitted himself . . . with commendable accuracy." Winston -- who would be a magnanimous victor -- thought that while England stood alone everyone in the nation should share his belligerent conviction that "A Hun alive is a war in prospect." On May 19, 1940, Clementine, attended the Sunday service at St. Martin-in- the-Fields and walked out when the preacher urged pacifism. Winston was outraged. He told her: "You should have cried, Shame, desecrating the House of God with lies!" Turning to Colville he said: "Tell the Minister of Information with a view to having the man pilloried." Jock would have been appalled six months earlier, but now he wrote in his diary: "It is refreshing to work with someone who refuses to be depressed even by the most formidable danger that has ever threatened this country."

COLVILLE's running account is a major primary source and should be the court of last resort for several outstanding issues. One is the charge that Churchill failed to warn Coventry of an imminent German bombing, though his cryptographers alerted him to it, on the ground that the Luftwaffe, realizing that its code had been broken, would switch to another. This is told in A Man Called Intrepid and The Ultra Secret, and it is an absolute fraud. "Intrepid" never met Churchill, never entered No. 10. Now Colville confirms the finding of the present writer, among others, that the P.M., having been told that the enemy's prime objective that Thursday would be London, spent the night in the capital. He learned of the destruction of Coventry the following morning.

On V-E Day a half-million Londoners, led by Churchill, roared these lyrics outside Buckingham Palace. Yet the Empire's knell had tolled from time to time during the war, and Colville reveals that Churchill had not been deaf to it. As early as January 16, 1940, a diary entry ends: "For some reason no subject is more boring to the average Englishman than the British Empire." On August 30, 1941, Winston learned that the Spitfires and Hawker Hurricanes which had won the Battle of Britain, driving Goring's bomber fleets out of the English sky, had been piloted by secondary schoolboys, not public schoolboys, and he said: "They have saved this country; they have the right to rule it." And on another occasion he told Jock he was reconciled to a postwar Labor government.

He was't really. He could never face a defeat with equanimity, and as the general election of 1945 loomed he once more became the old party warhorse. By then his place in history was assured, and his remarkable sense of humor kept him buoyant until the terrible depression of his last years. Even at the last, when he told Colville his death was imminent, he added, according to Colville's entry that day, that "he could not help wondering whether the government above might not be a constitutional monarchy." In the event, he said, the Almighty might "send for him" and ask him to form a government. He relished the prospect.