The Fringes of Power: <i> 10 Downing Street Diaries 1939-1955 by John Colville (Norton: $25; 796 pp.)</i>

From the warm, barely digestible glow produced on our side of the Atlantic by the visit of Prince Charles and Princess Diana, it would be easy to think of the British aristocracy as one indistinguishable cloud of privilege. In fact, this upper class is riven by a harsh class distinction of its own.

Primogeniture, that is. For centuries--until the Inland Revenue climbed into the act--it was a winner-take-all system. The eldest son got title and land. Daughters and younger brothers got crumbs. They could be such agreeable crumbs as a bishopric, an embassy or a place in a good regiment, and all kinds of parties and country weekends. But the earl’s younger brother was the eternal guest. He went to the parties; he did not give them.

This may seem a roundabout way of noticing the diaries kept by John Colville, private secretary to Winston Churchill through most of his time as prime minister. Their most obvious value is the precious view they provide of Churchill, and of Britain’s hair-raisingly improvised struggle in the first years of World War II.

But Colville himself starts out with primogeniture. His doing so is perhaps the key to this illuminating book. As much as anything, it is the Education of John Colville. It could almost be called “Bertie Wooster Grows Up.”

That “almost” is essential. Colville won first-class honors at Cambridge and flew as a fighter pilot for two wartime years after bullying Churchill into temporarily letting him go. Clearly, he is a man of brains as well as courage. Yet perhaps the brainiest, even the bravest thing he has done is make his diary what all great diaries must be: a record not of brillance willing to teach, but of foolishness willing to learn.

Through Colville’s own shifting perceptions, his diaries become the portrait of a ruling class tackling an ordeal it could not quite grasp at first. If Britain prevailed, it was with the help of Churchill--a man many regarded as a class traitor--a group of quite middle-class fighter pilots, a large national endurance, and the Americans and the Russians.

Colville was there as a privileged witness, and primogeniture helped put him there. His father was a younger son of a viscount; his mother, the daughter of a marquess. They lived quite well, but it was partly by scrimping and partly by invitation. Lady Colville wore her tiara to the opera but got there by bus and foot. Young John had to go to work.

His record at Cambridge got him through the Diplomatic Service exams. But the fact that for 30 years his mother was lady-in-waiting to the dowager Queen Mary--one of the above-mentioned crumbs--had more than a little to do with his being the particular young diplomat assigned to the prime minister’s office.

At the time, it was occupied by Neville Chamberlain. And the early diary entries show Colville as something of a Munich man. He fretted about the publication of a report on German concentration camps. Their revelations were “sordid,” he felt, and would stir people up on “evidence from prejudiced sources.” He shared the Conservatives’ suspicion of Churchill, then on the point of taking over, and sympathetically cites Rab Butler’s description of him as “a half-breed American” (his mother was American) and “the greatest adventurer of modern political history.”

And then, when Churchill swept in, everything was turned upside down. Sleeping in the basement of Downing Street, going through the bombardments, and living in the energy of a man who worked Herculean hours, and played in startling fashion, Colville spent his days in a fashion approximately like that of a lead atom in a cyclotron. He emerged as an atom of something far more agile.

In part, of course, it was a matter of being young and at work in the kind of emergency that disrupts all the normal ways of doing things. For a while, for instance, young Colville found himself in charge of appointing bishops. The prerogative was the Crown’s, but it was the prime minister who called the shots. With everyone so busy, the 25-year-old Colville, to his great delight, found himself walking across to Lambeth Palace to consult with the Archbishop of Canterbury on miter matters.

Increasingly, as he won Churchill’s trust and eventually affection, Colville became a witness to all the most central problems and decisions of the war. Several times, his entries revealed Churchill admitting that, apart from doing what was at hand, he hadn’t the faintest idea of how Britain could move from simply hanging on to actually winning. But he made living from moment to moment a feat so magnificent that it convinced his country and the world that there was much more to it than that.

Over and over, Colville diaries will describe graphically the blackness of the situation and the gleeful manner in which the prime minister went about inventing solutions. Nobody has bettered the description of him lying in bed late in the morning making decisions and announcing them to Mrs. Hill, his stenographer.

“Mrs. Hill sat patiently opposite while he chewed his cigar, drank frequent sips of iced soda water, fidgeted his toes beneath the bed clothes and muttered stertorously under his breath what he contemplated saying. To watch him compose some telegram or minute of dictation is to make one feel that one is present at the birth of a child, so tense is his expression, so restless his turnings from side to side, so curious the noises he emits under his breath.”

Churchill always did himself well. Carloads of work and visitors were transported on weekends to his country retreat, where the views were magnificent and the cuisine superb. The dinners would go on and on, and the conversation would proceed from the Greek idea of the citizen, to the habits of an Australian insect called the Cactoblastus--his guest was the Australian prime minister--to Charles de Gaulle giving superbly Gaullist advice on the proper behavior for an occupied people.

“The important thing was to remain aloof and superior. The Germans knew that they were inferior beings and were susceptible on this point.” And the evening would end with Churchill performing the manual of arms with an old hunting rifle.

The spirit of play, you realize in reading these diaries, was what allowed the British to get through their worst moment--Colville rode each morning in Richmond Park--and utterly to confound everybody else.