BEHIND every great man is a great friend. Genius such as that of the "man-child" Dr Johnson requires stimulation, generosity and forbearance to flourish. Hester Thrale might not have been an exceptionally gifted woman, but she did have an exceptional gift for friendship.

She was a precocious pet, born to loving parents of the landed gentry. Yet her childhood consisted of protracted visits to wealthier relatives as she and her mother waited and waited for her father to settle and make some money. In this tight but precarious family unit – dependent on Hester's likeability for their lodgings – Hester learnt the art of charm and companionship. How to talk. How to listen. How to respond.

Hester's conversation and kindness gave Dr Johnson refuge within her marital home. Although there were sexual undertones to Johnson's relationship with his "Dear Mistress", Macintyre suggests they were never physically realised. "Do not quarrel with your Governess for not using her rod enough," says one of her perfectly pitched responses to an overwrought Johnson.

This is an extremely readable and balanced biography, long but never slow. Surviving Georgian England was a risky business and the political uncertainties, violent uprisings, sexual adventuring and endless illnesses provide a taut read. It also reminds us how unhappy marriages brought endless, heroic stretches of child-bearing and child-loss. Hester gave birth to 12 children and watched eight die in infancy.

Hester kept a diary entitled Thraliana for 30 years, "a glorious ragbag" of personal, social and political commentary. Consider this, written when her ailing husband demanded to be taken on a logistical nightmare of a trip through Italy: "How shall we drag him thither? A man who cannot keep awake four hours at a stroke… if Mr Thrale dies on the road Johnson will console himself learning how it is to travel with a corpse."

Hester is studded with such wonderfully sharp words and prickly characters. However, the person I wished to hear more from was the author. I wanted more descriptive detail, atmospheric and actual, more definitive characterisation and thematic purpose: the passionate fascination Hester herself would have expressed.