Upmarket gossip columns (which prefer to call themselves diaries) had sport last week with the discovery that a second-hand copy of Gordon Brown's book on heroes, complete with signature, had been sold in a charity shop for a pound. It is comforting to know that, in an uncertain world, some things never change. Exactly the same anecdote was recited about the autobiographies of Ted Heath, Margaret Thatcher and John Major. I read the story with renewed pleasure at the thought that an active politician possessed both the inclination and ability to write about something other than himself. We live in an age in which the number of political memoirs grows almost as quickly as the quality deteriorates.
Years ago, Roy Jenkins asked me if I recalled that, in The Great Crash, Kenneth Galbraith had identified the beginning of the interwar slump as the day when the value of shares in Union Carbide plunged from several dollars to a few cents. I pretended that I did and he completed his story. A similar catastrophe, he said, had engulfed political autobiography on the day that Norman Fowler (sometime health secretary and subsequently chairman of the Conservative party) published Ministers Decide. We agreed that politicians who have never written much else often find that writing their memoirs does not have the effect on their reputations for which they hoped - as Tony Blair and John Prescott may soon discover.
As far as I can make out, Tony Blair has not yet announced that a duty to posterity obliges him to tell the true story of his Downing Street years - but he will. On the other hand, John Prescott - feeling understandably bitter at the treatment that he has received from a snobbish press - has spoken about "putting the record straight". Believe me, John, it will not turn out that way. Newspapers always want serial rights of autobiographies to deal with the subject the authors want to forget. Reviewers invariably revive old canards. Descriptions of real and considerable achievements are inevitably dismissed as special pleading and self-justification. Every political memoir is written in the hope that the most appropriate title will be Vindicated At Last. But that is rarely the result they achieve. The only sensible reason for writing a memoir - apart from the entirely respectable hope of earning money - is the intrinsic pleasure that the writing provides.
The reading public is wholly justified in treating with suspicion anything that retired politicians write about themselves. After a life dedicated to emphasising virtues and hiding vices, it is not easy to become honest to a fault and objective beyond doubt. I have spent much of 2007 working on what will become a biography of David Lloyd George. His monumental War Memoirs record years of epic achievement. But how would he have dealt with the Marconi scandal, the catastrophic outcome of the Paris peace talks, or the lies to the Sinn Féin leadership that secured the Irish Free State treaty but also precipitated the bloodshed in Ireland that lasted for the rest of the century?
During the five years in which I worked on The Edwardians and, more recently, the interwar years for Borrowed Time, I developed an immense admiration for one of the great radicals of British history. But I did not come to the conclusion that he could be trusted to tell the truth, the whole truth and nothing but the truth, about his life and times. Which politicians could?
Of course there are political autobiographies that, because they are worth reading, were worth writing. Harold Macmillan, Roy Jenkins and Denis Healey amuse, entertain, expose and reveal. They all possessed, as well as literary talent, enough self-confidence to protect them from the need to justify themselves to posterity. But in general, if it is acceptable history for which we are looking, we are not likely to find it in a retired minister's memoirs. So let us rejoice when active politicians prove that they have what Edna Healey calls a "hinterland", and write books about something other than themselves.
· Roy Hattersley's Borrowed Time: The Story of Britain Between the Wars is published on September 27