Few who celebrated Hogmanay in Edinburgh last week will have whooped it up like James Boswell, the biographer of Samuel Johnson. His newly published Edinburgh diaries paint the life of a man who couldn't stop drinking, being promiscuous or socialising with a judge who insisted there was a race of men with cats' tails, writes Kevin Barry.

At the wrong end of five bottles of claret and trying to ignore the siren call of the knocking shops, James Boswell tumbles through the tenement chaos of Edinburgh in the icy winter of 1774. "Much intoxicated," he writes in his diary. "Found myself . . . bouncing down an almost perpendicular stone stair. Could not stop but when I came to the bottom of it, fell with a good deal of violence, which sobered me much." Bruised and shaken, Boswell arrives home, in a wynd off the Royal Mile, and settles down to a long night with the quill and ink.

The model biographer, according to popular perception, should be a discreet and sober presence, steady of demeanour and subtle of eye, a neutral conductor who can draw out the vivacity of his subject and allow it to sparkle and fizz. The perception doesn't survive a reading of Boswell's Edinburgh Journals 1767-1786, in which the greatest of all biographers reveals himself as a rash and impulsive soul, easily foxed, fuzzy-brained, vastly bipolar and a martyr to booze, gambling and rabid fornication.

Here's another entry, from 1772, with Boswell shakily recording a fairly standard night on the tiles: "I drank too much. We had whist after dinner. When I returned to town, I was a good deal intoxicated, ranged the streets, and having met with a comely, fresh-looking girl, madly ventured to lie with her on the north brae of the castle hill. I told my dear wife immediately." His dear wife, the long-suffering Margaret, had heard it all before.

The journals, which were previously gathering dust at Yale, have been collated by Hugh Milne; they amount to a painfully truthful exposé of their lovable author's many quirks and foibles. We see Boswell bumbling about his business, as a legal advocate and in literary matters, forever grasping for the great achievement that might earn him the approval he desperately craved from the smart set of the London salons.

He never got a hold on it in his lifetime: the immortal Life Of Johnson, published in 1791, was for many years considered a crude and scurrilous frippery. With its scenes of elaborately rendered drama and reams of sparky quotations, it was a far cry from the staid chronicling of classic biography

But the Edinburgh journals, along with Adam Sisman's recent Boswell's Presumptuous Task, make a convincing case that, far from being the provincial groupie so derided by his contemporaries, Boswell eventually succeeded in the unthinkable and has in many ways supplanted his adored Samuel Johnson in terms of influence and achievement.

He formulated the conventions of modern biography and probably invented the celebrity profile while he was at it. Much contemporary features journalism can ultimately be blamed on the intemperate advocate.

Sisman has called the journals the workshop of a biographer in training, but they offer plenty more besides. In deliriously skewed detail, Boswell opens out the life of Edinburgh at the time - vivid and hectic and smelly, rumbling at a rolling boil - and we meet his reprobate coterie, a rum selection of oddballs and quarehawks, among them Lord Monboddo, a wondrously deluded judge who would solemnly inform a tavern gathering that on an island in the Bay of Bengal there existed a race of men with tails like cats.

Boswell and his fellow advocates enjoyed what comes across as a charmed existence. The daily routine centred on three elaborate and well-oiled meals, spread around the town's selection of fine dining rooms, and the legal gentlemen would conduct most of their business in the frothy ale houses. Points of law would be laid aside as the lawyers whispered about accommodating floozies and bemoaned their latest sexually transmitted diseases.

For all his posturing and divilment, however, Boswell emerges as a demon worker. Despite his frequent "visits from Signor Gonorrhoea" and his many bouts of black melancholy, he put in the hours. Towards the end of these journals, after Johnson has ascended to the great coffee shoppe in the sky, a London publisher asks Boswell to knock out a quickie book on his friend. He refuses, and instead spends seven hard and poverty-stricken years on the Life, frequently ignoring the wails of his dying wife as he ploughs on with the writing.

There's a lot of sadness in the journals. Boswell tries to curb his drinking, and fails. He swears off the whoring, and fails. He tries to live within his means, and fails. The failures thicken up and leave him embittered and a wreck. But you're left with the feeling that, despite the twittering and rashness, there was a strange though unmistakable dignity to the life of Boswell.

Boswell's Edinburgh Journals 1767-1786, edited by Hugh M. Milne, is published by The Mercat Press, £12.99 in UK