Samuel Pepys: the unequalled self, by Claire Tomalin

Diana Souhami assesses this leading light of Restoration England

This boook is a great achievement and a huge pleasure. The "unequalled self" of the subtitle is from an essay by Robert Louis Stevenson commending the "unflinching sincerity" of self-disclosure in Pepys's diary. Pepys watched himself behaving well or badly and, for 10 years from 1660, wrote the details down. Claire Tomalin, in her turn, applies unflinching scrutiny to the whole of his life.

Her research is meticulous. For example, in her acknowledgments she thanks the Real Tennis Club of Cambridge for giving her the diameter (2in) of the real as opposed to modern tennis ball, which Pepys's kidney stone (removed in May 1658) equalled. Her description of his surgical process is awful in its vividness. Palliatives of oil of earthworms, cinnamon and chicory make matters worse. Such detail illuminates the book. Pepys recorded an incident in April 1664 when he chanced on his boss's wife, Lady Sandwich, as she sat on a chamber pot. She blushed, Pepys talked hastily. With Tomalin we revisit the scene as modern tourists. She invites us to be startled, amused and quickly to move on. She is keen to reveal everything there is to know about the inner man and his outer world.

Hers is an engaging juxtaposition of author and subject: a bookish, respectable 21st-century woman and a short, pop-eyed, 17th-century fellow in a periwig, who all too often had his hand up the skirts of a teenage maid, or was quarrelling with colleagues or wife. There are centuries of time to separate them. Shared humanity is their common ground.

Claire Tomalin, we are sure, does not spit black phlegm, go to executions, piss in a chamber pot or beat servants with a broom. But she is very loving to Pepys; not uxorial, more a devoted bodyguard. She is gutted that an early fictional work of his has been destroyed, and sympathises with his bladder problems. His energy "burns off blame, making it surprisingly hard to disapprove of him".

Her admiration for him is as a diarist of genius. This "secret masterpiece" places him, in her view, alongside Milton, Bunyan, Dickens and Proust. She describes him as "the most ordinary and extraordinary writer we will ever meet". Part of her intention is to direct her reader to his work.

Pepys began the diary on 1 January 1660, with the entry that his wife, Elizabeth, was not pregnant. He was 26, she was 19, and they had been married five years. It troubled him that they remained childless, even after attempts to combat infertility by drinking sage tea and changing the level of the bed so their feet were higher than their heads. Their marriage was at all times problematic. Both had tempers that flared to violent rows. She had some sort of vaginal boils; he, recurring gall stones. She suspected him of infecting her. His last entry was on 31 May 1669. In the preceding weeks his wife had found him with her maid, his "hand in her cunny". She threatened to shame him, waved red-hot fire tongs at him, kept him awake raging at him. He stopped the diary out of fear for his eyesight, and said that abandoning it was a form of death.

It had been his haven. He ruled the margins in red, wrote in ink with a quill pen, spaced the lines evenly, devised a shorthand for his sexual goings-on with tavern girls and chambermaids. His themes were his career, money, domestic experience, books, theatre, music, sex.

He lived through the confusion of England as a republic and the restoration of the monarchy. He rose from being a poor clerk to a "thriving condition" as a naval administrator. He covered events of huge impact. In the plague year of 1665 he proved immune to infected fleas, chronicled the death of Londoners, work, family quarrels, and his fondling of Mrs Bagwell and Sarah, the girl at the Swan Inn. In the days of the Great Fire, he buried his wine and Parmesan cheese, took his bags of gold, accounts and diary to a house in Bethnal Green, saw looting and a singed cat rescued from a chimney, and 400 streets reduced to smoking ruins.

He was with the fleet of ships that brought Charles II back, triumphant, from the Hague. He procured the "rich barge" to bring the king ashore and the musicians to fanfare him.

Pepys's diary is history and comedy, particular in its detail, universal in its range. Tomalin talks of the "bursting, disorganized, uncontrollable quality of his experience". Her achievement is to organise, control and give it context, without tarnishing its shine. All her excellent research is lit by Pepys's self-revelations. Her biography develops into a vivid chronicle of contemporary history seen through the all too human preoccupations of this ordinary, and extraordinary, man.

Diana Souhami's 'Selkirk's Island' won the Whitbread Biography Award.

Lisa Jardine and Claire Tomalin will be appearing at the Cheltenham Festival on 13 and 11 October respectively