Hester Thrale was a political wife before the term was invented, a landowner at a time when wives could not own land in their own name and, above all, a diarist and author. Her ambitious final book, Retrospection, was the first attempt by an Englishwoman to write a history of the world.

Hester Salusbury married Henry Thrale in October 1763, shortly after her father's death had severely reduced her options. Eleven months later she gave birth to their first child and from then on was pregnant almost yearly during her marriage to Thrale. He forbad her from riding - too masculine; and from entering the kitchen - too smelly. She was saved from a life of stultifying boredom by the arrival for dinner in 1764 of Samuel Johnson, then 55, widowed, living in squalor and close to a mental breakdown. Yet, in spite of his dirty appearance, scrofula and scarring, Hester and Johnson took to each other. He soon considered the Thrale home his own and encouraged her to write more poetry.

How this young woman came to rescue Johnson and in turn find her own strong literary voice forms the core of this biography. For 16 years she provided him with endless cups of tea and stimulation as they debated politics, child rearing, world affairs and literature. But the book's interest goes far beyond the deep friendship between Hester and Johnson. Hester Thrale, bluestocking and wit, was a remarkable woman imbued with deep intellectual curiosity revealed in six leather-bound volumes now housed in the Huntington Library in California, known as Thraliana. The blank books, a gift to her (and posterity) from her husband on their 13th wedding anniversary, were eventually filled with detailed accounts of domestic politics, the French Revolution, cameo portraits of friends and enemies as well as Latin epigrams, gossip, poetry and such fascinating details as the price of a shirt in 1801, shedding a powerful beam of light on the life and culture of Georgian England.

She put up with death, illness and her husband's frequent infidelities in a way that indicates that this was nothing more than the age expected. Aged 38, after a stillbirth that nearly killed her, Hester nonetheless hoped that a visit to his mistress might “dissipate” her husband's gloom.

By contrast, some of her preoccupations seem very modern, especially her desire to find emotional satisfaction through romantic love. Her second marriage, to her daughter's Italian music master, Gabriel Piozzo, gave her renewed enthusiasm for writing but lost her Johnson's admiration and blighted relations with her daughters for the rest of her life. McIntyre's detailed account of their shabby treatment of Hester makes painful reading.

This entertaining book brings her out of the Johnsonian shadow at last and Hester is revealed: a heroine for any age.

Hester: The Remarkable Life of Dr Johnson's 'Dear Mistress' by Ian McIntyre
Constable, £25; 320pp Buy the book

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