Margaret Thatcher: The Authorized Biography; Volume One: Not for Turning, By Charles Moore Not for Turning: The Life of Margaret Thatcher, By Robin Harris

The late premier's chosen biographer has done a great job. She did have secrets to tell, and to hide

After the triumphant obsequies comes the pent-up rush of new biographies, "authorised" and otherwise. Charles Moore was appointed Lady Thatcher's official biographer in 1997, on condition that the book did not appear in her lifetime.

He might reasonably be a little miffed that the Lady also approved another life by Robin Harris, who helped her write her memoirs, now published with the same title and a prominent endorsement. Harris has written a lively, loyal and unashamedly admiring study of his heroine ("The reader will discern remarkably few warts") with the great benefit of being single-volume. Nevertheless, the main event is unquestionably Moore, who has discharged the first part of his commission superbly.

He has marshalled a huge range of sources, many of them new, without letting himself be swamped. In addition to Lady Thatcher's own papers, with her famously furious scribblings all over the submissions by ministers and officials, he has quarried the National Archives, Conservative Party and American archives (where he acknowledges some research assistance: otherwise he commendably did most of it himself).

He has spoken to practically everyone who ever had anything to do with her, and interweaves their recollections skilfully to bring out wider themes. He has also been given access to private diaries which bring immediacy to the narrative. These are the benefits of being the official biographer.

Above all, in this first volume, he has the young Margaret's letters to her elder sister Muriel. Mrs Thatcher's own account of her early life, later expanded in her memoirs, while not untrue, was not the whole truth. The one person who could have told more about the real dynamics of the Roberts family above the shop in Grantham was Muriel. But she discreetly never talked.

Moore had "two substantial conversations" with her before she died in 2004. He still did not get much out of her about family life – though she did suggest that their hitherto shadowy mother was the stricter and more puritanical of their parents. But he does have 150 letters Margaret wrote to Muriel, as well as some later ones between Muriel and Alfred; these are a goldmine.

First, they reveal the young Margaret to be obsessed with clothes: she is always telling Muriel what she wore to every dinner, ball or meeting she attended, and how she managed to buy, borrow or adapt her outfits for each occasion. Yet she was not concerned with clothes for their own sake, but specifically for the effect they enabled her to make. From an early age she loved turning heads. More clearly than before one can see how knowingly, as PM, she used her clothes to dramatise her impact.

Second – and completely unknown – they reveal that she had several quite serious boyfriends before she married Denis, all but one considerably older than herself. Yet there is no great passion in her regard. She appraises them coolly as husband material, so that the decision to accept Denis ("not a very attractive creature"), rather than a 47-year old doctor by whom she was a good deal more smitten, seems more than ever a calculated career move.

Much later, Alfred's lonely letters to Muriel confirm that Margaret had very little contact with her sainted father after she moved south and launched on her career. There is some doubt she even attended his funeral. She rediscovered and revered his memory only when he was safely dead and she needed to reinvent herself as the grocer's daughter. These family letters, while they do in some ways render her more human, reinforce the view that she was from the beginning chillingly focused on her career.

From Oxford onwards, the story of her struggle to impose herself in a man's world is broadly familiar, but told with a new depth of detail. Love her or loathe her, it is a heroic odyssey: her phenomenal determination and inexhaustibility, her combination of blazing certainty with intense practicality, leave the reader repeatedly awestruck. Moore manages to demonstrate all these qualities without being in the least bit hagiographic.

This volume covers the first three tumultuous years of her government. Moore does not shirk or simplify the complexity of the issues she faced: not just the economy, inflation, the nationalised industries and the trade unions, but also Rhodesia, Northern Ireland, Britain's European budget contribution and lesser episodes like the Iranian embassy siege and the unmasking of Anthony Blunt.

All these, but above all the agonised arguments between her advisers and gurus - far more important than most of the Cabinet – over taxation, exchange controls, monetary targets and Geoffrey Howe's 1981 budget, are recounted with exemplary clarity, not just through her eyes but with complete fairness. More than just biography, this is excellent history. The level of tension within No.10 at the nadir of her popularity is captured by an extraordinary memo the head of her policy unit, John Hoskyns, wrote in August 1981.

Hoskyns believed passionately in what she was trying to do, but was afraid that her style of management endangered the enterprise. Bravely – he was also an ex-soldier - he told her so. "You lack management competence... Your own leadership style is wrong... You bully your weaker colleagues. You criticise colleagues in front of each other... You give little praise or credit and you are too ready to blame others... The result is an unhappy ship". She never changed: if anything she got worse. You can already see why they got rid of her in 1990.

The last hundred pages are devoted to the Falklands: an accidental episode, marginal to her real project, which nevertheless became her apotheosis and earned those military honours at her funeral. Moore ends with the victory dinner which "may well have been the happiest moment of her life". If the second volume, charting her mounting hubris and eventual nemesis, maintains this quality it will be a tremendous achievement.

John Campbell's biography of Margaret Thatcher, 'The Iron Lady', is published by Vintage