It also shows the personal toll the 1982 conflict with Argentina over the Falkland Islands took on her; and reveals the secret talks she allowed government officials to hold with the IRA.
Thatcher chose journalist Charles Moore to write her life story, giving him unique access to a treasure trove of personal and government papers on condition the book was released after her death.
It charts her childhood in the east England market town of Grantham and her first romance, which started while she was a student in wartime Oxford.
It follows her rise through the political ranks up to what many consider her finest achievement, the successful defence of the Falkland Islands in the 1982 conflict with Argentina.
A second volume will cover her later life in government and her retirement.
Moore's book describes how Thatcher, who died on April 8 aged 87, was not entirely impressed with future husband Denis on their first meeting.
He is, she wrote, "not a very attractive creature - very reserved but quite nice".
She described the former army captain as aged "about 36, plenty of money" - and unpopular with workers at the paint works he ran as he was "far too belligerent in dealing with them".
The book reveals how Thatcher set up her sister Muriel with future husband Willie Cullen despite dating the farmer herself at the time.
Cullen plied the future prime minister - then known as Margaret Roberts - with gifts including "frightfully expensive" Crepe de Chine perfume and a "very nice" handbag with her initials on it.
But even as he courted her, Thatcher was writing to her sister: "Went to the flicks yesterday with my farmer friend and got him all primed up to meet you sometime.
"I showed him the snapshot of you and I together - and he said he could scarcely tell the difference so I should think we could easily substitute me for you. When can you come down for a weekend?"
Cullen was eventually introduced to Muriel on April 8, 1949, and the pair announced their engagement on Valentine's Day 1950.
The book, entitled "Margaret Thatcher - The Authorized Biography: Volume One: Not For Turning" gives a detailed account of her rise through the ranks of the Conservative Party.
It shows the battles she faced as a woman in the male-dominated world of politics.
One correspondence described how a male cabinet colleague described her as that "bloody woman."
"Her job is to sign them (letters), not read them," he added.
Another passage reveals Thatcher's ignorance of the machinations of government despite her ascension to the top job in 1979.
On her arrival in Downing Street she turned to senior civil servant Ken Stowe and asked: "Ken, what do I do now?", to which he replied: "You've got to form an administration".
The book also recounts how Denis found his wife sobbing on the end of their bed saying "Oh no, oh no. Another ship! All my young men!" after a night of heavy British casualties during the war against Argentina to reclaim the Falkland Islands.
It explains too how she gave the green light to secret contacts with the IRA in 1980 over conditions at the notorious Maze prison and during the 1981 hunger strikes, despite publicly vowing not to deal with terrorists.
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