Ruskin and Venice go together like Romeo and Juliet. They were passionate, fated lovers. The affair between the great Victorian critic and La Serenissima was to produce something of lasting value, as indeed did the tryst between Shakespeare's fictional Italian teenagers  a love story that haunts us still. To Ruskin, though, we owe a different kind of love: the conservation of that haunting Byzantine Venetian basilica, St Mark's.

Without Ruskin and his passionate rhetoric, St Mark's might well have been restored to some soulless "as new" condition by his French bête noir, the gothic revival architect Eugène Viollet-le-Duc, whose approach to restoration the Englishman skewered. And the subtle conservation of St Mark's led in turn to the founding of William Morris's Society for the Protection of Ancient Buildings, and, one way or another, to the principle of "anti-scrape" architectural conservation not always adhered to, yet at least paid lip-service to, throughout Europe today.

As for Ruskin, he last visited Venice, the city he loved so deeply and shaped in our collective imagination, in 1878. By then he was a human shipwreck. Madness claimed him, and for the last decade of his life, spent overlooking Coniston Water rather than the Grand Canal, he disappeared into an unfathomable silence.

Robert Hewison's truly first-class book  generously illustrated and handsomely produced  is the product of research carried out with forensic rigour over 40 years. It demonstrates with lucidity and conviction how Ruskin reinvented Venice both for himself and for us. The Venice we visit today, wishing away the crowds, is still largely his.

When, in 1835, he first came to the city with his strict yet loving evangelical protestant parents, Ruskin saw it mostly through the romantic and lustful eyes of Byron. It was, perhaps, almost impossible even then for a visitor to see Venice any other way.

Hewison shows, in scrupulous and engaging detail, how Ruskin learned to shrug off Byronic fictions and to see the city afresh. Because he believed that the medieval history of Venice had something profound to tell contemporary British society  here had been a world of wise and moral governance expressed through honest and magnificently crafted gothic, and thus Christian, architecture  Ruskin set out to write the city's story stone by stone. The Stones of Venice was a labour of love that was to have powerful effects on both British architecture of the neo-gothic era and on the fledgling conservation movement championed by Morris.

Love was certainly involved in the book's making. In a note written in 1841 the 22-year-old Ruskin makes the telling admission, "the outlines of St Mark's thrill me as if they had been traced by A's hand." A was Adele Domecq, the beautiful daughter of John Ruskin senior's partner in the noted wine business, Ruskin & Domecq. If John had hopes of marrying Adele, these were dashed by his parents: the idea of their beloved son wedded to a Roman Catholic was as inconceivable as it was abhorrent.

Ruskin, though, conflated his vision of Venice with the love of a woman. And women. As Hewison conjectures, the critic's vision of the city he came to know as intimately as lovers know one another's bodies was erotically charged. And yet  and this seems so very sad, and led no doubt to Ruskin's madness  he was unable to form a full, loving relationship with a woman, whether Adele, Effie Gray (his wife) or, in later years, Rose la Touche. The deep-seated problem in Ruskin's psyche was the fatal mismatch between his evangelical protestant conditioning and the effect of the Catholic city he loved.

While his young wife, Effie, flirted with dashing Austrian officers in Venetian ballrooms, Ruskin clambered over the stones of Venice, measuring tape in hand. "The rate at which Venice is going is about that of a lump of sugar in hot tea," he wrote to his friend, the painter George Richmond. Ruskin, however, dissolved, too. Venice began to mirror his encroaching madness. The city was also to become, as he fretted, anathema to him. His Stones of Venice had encouraged what were, to him, the absurd Venetian-gothic designs of the gasworks of Brentford and the factory chimneys of Croydon.

And yet, as Hewison so astutely documents and analyses, Ruskin's affair with Venice was also to give us the critic's social commentary that was to affect British life after his death in the guise of national insurance, national health, universal free education and even the National Trust. Ruskin's gothic Venice was built, as he saw it, by craftsmen working creatively for a moral, purposeful republic aiming for the best in all things. Britain could, and did  through Ruskin's eyes  learn from it. He remains, for all his flaws and ultimate madness, the guardian angel of both Venice and our own sodden islands. Hewison resurrects this passionate thinker and charismatic writer with the care, and love, that is surely his due.

Jonathan Glancey's Spitfire: The Illustrated Biography is published by Atlantic.