Samuel Johnson was one of the scruffiest men in the history of English literature. His clothes were filthy, his wigs didn't fit, his shoes were in tatters. In physical terms, he was gross, even terrifying. Scarred by scrofula, painfully short-sighted (almost blind in his left eye), quite deaf, he was huge, big-boned, greedy: his table manners were disgusting, he pawed his food, he guzzled 17 cups of tea at a sitting. Indeed, whenever tea appeared, said Sir John Hawkins, one of his first biographers, he was “almost raving”. James Boswell, his best-known biographer, described him on first meeting as “a man of most dreadful appearance”. He spoke with “a most uncouth voice” and, said the bookseller Tom Davies who introduced Boswell, had a laugh like a rhinoceros.

Johnson was born, by his own account, “almost dead”. His Lichfield wet nurse gave him scrofula, but where his Tourette's came from, if it was Tourette's, biographers can only speculate. From an early age he displayed an extraordinary range of tics. The novelist Fanny Burney describes his mouth as “almost continually opening and shutting, as if he was chewing. He has a strange method of frequently twirling his fingers, and twisting his hands. His body is in continual agitation, seesawing up and down; his feet are never a moment quiet; and, in short, his whole person is in perpetual motion”. Sir Joshua Reynolds's painter sister Frances details the “scarcely credible” antics of his hands and feet - sometimes as though he were “holding the reins of a horse like a jockey on full speed”.

He tended not to start conversations, but to descend into them, often from a height. He was dogmatic, overbearing, irresistibly articulate. His vehemence came young, in Staffordshire. He deplored the inefficiency, and near insolvency, of his bookseller father - a depressive whose black dogs he inherited. He fought a lifelong battle against “indolence”, and dreaded the onset of madness. Arrogantly precocious, he crossed swords with his schoolmasters. When he tried to be a schoolmaster himself, he was turned down because of his bizarre appearance. He resented his poverty, and could never quite forgive one of his few pupils, the future actor David Garrick, for becoming so rich when he remained always so poor.

If Johnson had not written a word, he would still make a wonderful biographical subject. As it is, he towers over the 18th century - the single-handed architect of the huge and innovative 1755 Dictionary of the English Language; the editor and champion of Shakespeare; the essayist of The Rambler and The Adventurer, “The Idler” in the Universal Chronicle; the poet (author of The Vanity of Human Wishes), playwright, journalist, pamphleteer and epitaphist. He embodied supreme self-confidence, based on a belief in God and self-education. When asked why he had given a wrong definition in his Dictionary, he replied unfazed, “Ignorance, madam, pure ignorance.”

Peter Martin makes a case for him also as a biographer. The “biographical part of literature,” said Johnson, was “what I love most.” He was less interested in the acts of heroes than in tales of common humanity - he wanted to know “not how any man became great, but how he was made happy”. The best biographies were those to whom the writer brought “his own story”. Boswell, whose life Martin has also written, said of Johnson that he “excelled” in biography “beyond all who have attempted that species of composition”.

Boswell was 22 when, in May 1763, he first met Johnson in Tom Davies's bookshop. Johnson was 53, with all his leading work, apart from his Lives of the Poets, behind him. For the next 21 years, Boswell made Johnson his project, not only recording conversations, but also provoking him to fury with his interview technique. The Life of Samuel Johnson, published in May 1791, six years after Johnson's death, is a masterpiece, to which Boswell brought his own story and subtle art. It consolidated the legend of the “Great Cham”, a conversational gladiator - as already retailed in the 1786 Anecdotes of Johnson's close friend Hester Piozzi, a sprightly portrait based on her notebooks, and Hawkins's 1787 Life, damned by Boswell as ponderous.

The challenge for all later biographers has been to reclaim the writer from the anecdotage. Martin is industrious and engaging, if nervously 21st-century (does he need to say that Johnson would have been a “popular guest” on television chat shows?), but gives us little new. James L Clifford was an incisive scholar; John Wain imported his own Staffordshire story; Richard Holmes has written a brilliant cameo; Walter Jackson Bate's 1978 life remains majestic. Martin's virtue is his focus on his subject's humanity: less on the public man, more on his strange character, his depression and physical awkwardness, his piety and generosity - his brave tussles for happiness. And for Johnson that was the recipe for good, accessible biography.

Samuel Johnson by Peter Martin
Weidenfeld £25

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist