On 4 April 1955, after entertaining the Queen, Prince Philip and other guests to dinner on his last night in Downing Street, Winston Churchill retired to his bedroom. He sat silently on his bed, still wearing his Garter, Order of Merit and knee-breeches, for several minutes. Then he said vehemently: "I don't believe Anthony [Eden, his successor, anointed by Churchill himself] can do it." We know this from the diary of Jock Colville, his long-serving private secretary, who guided the old man to bed. Colville observed: "His prophecies have often tended to be borne out by events." And so, by general consent, it proved in this case, with Eden's premiership collapsing, after little more than 18 months, in the Suez disaster.

Towards the end of this book, Ruth Winstone, who has previously edited three volumes of diaries by the former Labour MP Chris Mullin and an exhausting nine volumes by Tony Benn, speculates that "the political diary may well have reached its natural end." Blogs and social networking sites, she suggests, have replaced "an essentially reflective private activity  with instant communication, shared with hundreds of thousands of people". Her book  a collection of brief diary extracts from 75 authors including Colville  shows why she is wrong. It is impossible, even if the means had been available to him, to imagine Colville tweeting Churchill's bedtime reflections. Today, backbench MPs and some ministers may tweet and blog, but their outpourings rarely rise above the mundane. Even if publication is premeditated  as is clearly the case with most recent examples  political diaries, unlike memoirs, give a sense of history behind the scenes, of how the powerful gossiped, plotted and bitched about each other as events unfolded, of their prejudices, their feuds, their obsessions, their sometimes foolish hopes.

There is much to enjoy in this book. It is always a pleasure to be shocked and dismayed over the prejudices of our ancestors, even though some distantly echo those that are widely held now. Beatrice Webb, a founder of the London School of Economics and the Fabian Society, and married to a Labour MP, mused in 1922 on whether when English children were "dying from lack of milk", one should extend "the charitable impulse" to Russian and Chinese children who, if saved this year, might anyway die next. Besides, she continued, there was "the larger question of whether those races are desirable inhabitants" and "obviously" one wouldn't "spend one's available income" on "a Central African negro".

Hugh Dalton, offered the Colonial Office during Attlee's 1945-51 Labour government, turned it down because "I had a horrid vision of pullulating, poverty stricken, diseased nigger communities, for whom one can do nothing in the short run and who, the more one tries to help them, are querulous and ungrateful." Harold Nicolson, soon to join the Labour party and become a parliamentary candidate, told the editor of the Times, during lunch at their club in 1946: "our lower classes are  congenitally indolent  only the pressure of gain or destitution makes them work." Virginia Woolf, novelist and Labour supporter, was shocked in 1929 when her servant, Nelly, said "We are winning." It had not previously been clear to her (or, indeed, to many other well-born early socialists) that the Labour party had anything to do with servants, and she didn't, she confided in her diary, want to be ruled by "Nelly and Lottie" (another servant).

Yet if Woolf's fear of servants' rule seems to belong to another world, concern over the power and conduct of the press doesn't. Churchill's reflections on Lord Beaverbrook, proprietor of the Daily and Sunday Express, were recorded in 1945 in the diary of his doctor, Lord Moran. "He has made several governments in my lifetime. It's better to have his support than his opposition." Harold Macmillan, just after leaving the premiership in 1963, found it "wonderful not to read the newspapers" with their "steady stream of falsehood, innuendo, poison" written (and here the diarist betrays the Edwardian attitudes he never quite shook off) as though Britain were "a vast servants' hall", interested only in gossip. On the other hand, Richard Crossman, writing in 1952, thought press power had declined since the first world war, when Lord Northcliffe (who then owned the Daily Mail, the Observer and the Times) "could really make and unmake cabinet ministers". Labour cabinets, he thought, were "not nearly as open" as Tory cabinets to press influence, because they wrote off nearly all newspapers as capitalist.

Winstone has a good eye for such passages, showing how previous generations of politicians saw issues that still preoccupy us. Particularly revealing is a lament from Macmillan in 1952 that, "in deference to modern political thought" (or what would now be called "political correctness"), his local church had omitted the third verse of "All things bright and beautiful": "The rich man in his castle / The poor man at his gate / God made them high or lowly / And ordered their estate." Winstone also picks out unexpected juxtapositions, bringing together the old world and the new as the 1960s revolution progressed: the 57-year-old Malcolm Muggeridge meeting the Beatles in Hamburg in 1961 ("weird feminine sounds"); the 66-year-old Cecil Beaton, the royal family's favourite photographer, taking LSD in San Francisco in January 1970 ("I felt a bit quavery  and I sweated"). And she shows how a diary entry can illuminate, as nothing else, the shock of great events to those who learned of them for the first time. Moran, told by Churchill at the wartime allies' Potsdam conference in 1945 that "we have split the atom" and the bomb would be used on Japan, reflects: "I once slept in a house where there had been a murder. I feel like that here." On 22 November 1963, 76-year-old Violet Bonham Carter, daughter of the early 20th-century Liberal prime minister Henry Asquith, goes to see the Boulting Brothers film I'm All Right Jack, misses her stop on the bus home and walks back a mile and a half. When she gets home, she learns of President Kennedy's assassination. "I felt a personal stab of shock and horror  The west is orphaned. The world is orphaned."

Yet for all its occasional delights, this book doesn't quite work. On the cover, the historian David Kynaston compares it to "a great rolling novel". That is exactly what it isn't. Telling the history of the period from the 1920s to the end of the new Labour government in 2010 through diary extracts may have seemed a good idea, but the effect of hearing from so many different witnesses running on and off stage is oddly disorienting, like those old Saturday afternoon TV sports programmes where you were switched from snooker to horse racing and back again. Too many rather dull entries are there merely to record events. From Pepys onwards, the best diarists have held their readers as much through what they reveal of their own characters as through their accounts of events. There is some of that here: the extracts from Benn's diaries, for example, reveal how he has always been as earnest about technology (he is thrilled by the arrival of the telephone answering machine and proudly records that he has "saved" the Harrier vertical take-off jet) as he is about politics. But Alan Clark's political observations, wrenched from the context of the author's rampant libido, can seem surprisingly narrow and banal.

The book probably belongs in the downstairs loo, to be picked up briefly and dipped into at random. If it encourages more people to read the full published diaries of, say, Harold Nicolson, Henry Channon, Chris Mullin and Barbara Castle, as well as Clark and Moran, it will have done its work.

 This article was amended on 7 January 2013. The original said that in 1946 Harold Nicolson was "soon to join the Labour party and become an MP". He was soon to become a parliamentary candidate for the Labour party, but he lost the subsequent byelection.