A few days after Margaret Thatcher was elected leader of the Conservative Party in 1975, she headed north for an engagement in Edinburgh.

In later years, her visits to Scotland were always hostile affairs, but on that first trip, the welcome was warm. On Princes Street, a large friendly crowd pushed and shoved to get a closer look at this strange creature – Britain's first female political leader – but what was stranger still was the fact she was a Tory. The oldest, stuffiest party had done something truly radical (without really meaning to) and now here she was, in matching hat and shoes, on Princes Street. She had to take refuge in a shop to escape the crush.

The fact that Charles Moore in his authorised biography of the late Baroness Thatcher has managed to uncover this surprising little anecdote, after several weeks in which it feels as if every fact and story about her has been disgorged, is impressive. In the flyleaf of the book, Moore seeks to assert his objectivity on his subject but it's clear he also wants to use his heavy 850-pager to dispel a few myths. And he does. Lady Thatcher did get tired, he says; she did have doubts; she did have to compromise, particularly in the early years at Number 10, when she was, in Moore's words, trapped in moderation.

Most of the new material Moore uses comes from around 150 letters Lady Thatcher wrote to her sister Muriel in the years before power. This means the first half of the book, which is largely based on the letters, is much more satisfying than the second which, focusing as it does on the years 1975 to 1982, relies more heavily on material that has been in the public domain for some time. What the letters reveal is that Lady Thatcher never at any point sat down and thought Thatcherism into existence. In all the surviving correspondence from the 1940s, there is not a single instance of her expressing a political view and the reason for that is she already knew what she thought. She had worked it all out, she says, before she was 17. She was a person of beliefs, not ideas, and her early career was a search for the words to express those beliefs.

Most of the beliefs came from her father Alfred Roberts, a Methodist preacher and mayor of her hometown of Grantham. Lady Thatcher often praised her father in public, but what emerges from the letters and papers Moore has used (and this is an authorised biography) is that, in later life, her father felt neglected by his famous daughter. There are also suggestions that Denis Thatcher struggled with the fact his wife was so absorbed in her career. In the 1960s, he had a nervous breakdown and went to South Africa to recover. The implication is that he might not have returned to his wife. Fortunately for Lady Thatcher, he did.

This detachment from family life lends a melancholic air to large sections of Moore's book, but he isn't judgmental and leaves us to make our own decisions about whether Lady Thatcher's commitment to her career rather than her family is admirable or shocking. When the Thatchers left for work in the morning, for instance, Denis would always remember to wave up to the nursery window. His wife, on the other hand, would always forget because her mind was already on work. And once there, she was impressive, committed and hard-working. One parliamentary private secretary whom she ordered to redraft some letters was overhead to remark: "Bloody woman. Her job is to sign the letters, not read them."

It is easy to forget how unusual all of this was – a woman in command in politics – and hard to remember that she faced patronising and sexist attitudes from another age. When Harold Macmillan made her a junior minister, for instance, he told her she should wander in around 11am, look around and then go home. "I shouldn't stay too long," he said. And then there was the newspaper headline on the day Lady Thatcher was chosen for the Finchley constituency: "Tories Choose Beauty."

Moore puts all of this into its proper context even if he does sometimes sacrifice colour and drama for facts, facts, facts. He certainly leaves us with a strong impression of Lady Thatcher as someone who was always unfashionable, a confident misfit, an outsider and a careerist who was terrified of being bored. This book is written by someone obviously sympathetic to her but its conclusion, squeezed from papers, letters and interviews with a woman reluctant to give up any facts about her personal life, feels right: Thatcherism was never a philosophy. It was an instinct – loved and hated – that was embodied in a highly unusual woman.

Margaret Thatcher: The Authorised Biography – Volume One: Not For Turning

Charles Moore

Allen Lane, £30