As he releases himself from his everyday levity to study an early mosque or tomb, one can positively feel the tensing of critical muscle and the rapt responsibility to his subject. Confronted by the handful of buildings to which he attributed genius—the minarets of Herat, the Gohar Shad mosque in Meshed, the Sheikh Lutfullah in Isfahan, and finally Gumbad-i-Kabus—his vocabulary expands into a lexicon of striking colours: café-au-lait, gentian, prawn, the bloom of grapes or peach.

His obsession with architectural brickwork was shocked into second place by the experience of Herat, a city ruled throughout the fifteenth century by the civilized descendants of Tamerlane. The lonely minarets of its near-vanished royal college, built by the patron queen Gohar Shad, converted him to the beauty of tiled decoration, and his description of these masterpieces preserves them more richly than any photograph. But it is a bitter memorial now. Since Byron’s day the two finest minarets have fallen, and the others are so shaken by Russian gunfire that their mosaic faience cover the ground in multicoloured pools.

In this man of extremes, the counterpart to his architectural loves was, of course, the repudiation of received opinion, and of sentimental orientalism (“the Omar Khayyam brigade.”) As he admired his first Seljuk mausoleum he exulted that “This at last wipes the taste of the Alhambra and the Taj Mahal out of one’s mouth… I came to Persia to get rid of that taste.” As for the giant Buddhas of Bamian, his description may go a small way to reconciling a reader to their destruction by the Taliban: “Neither has any artistic value. But one could bear that; it is their negation of sense, the lack of any pride in their monstrous flaccid bulk, that sickens.” In his diary he concluded: “If I lived with the Buddhas, I should be ill.”

Typically, he traced the vigour of Persian Islamic art to a time well before its supposed zenith in the sixteenth century, finding its roots in his Turkic and Mongol brick tomb-towers. Of these the high, phallic mausoleum of Gumbad-i-Kabus, whose dead prince had been suspended there in a glass coffin in 1007, was the apogee. “A tapering cylinder of café-au-lait brick springs from a round plinth to a pointed grey-green roof, which swallows it up like a candle-extinguisher… Up the cylinder, between plinth and roof, rush ten triangular buttresses, which cut across two narrow garters of Kufic text…”

It was the passion for such monuments that drove Byron forward. An intermittent hypochondriac, he hated what he called “adventure”. Yet twice, endangering his life, he entered the forbidden mosque of Gohar Shad in disguise, his face blackened by charcoal; and his journey from Herat to Mazar-i-Sharif—he and Sykes were the first Englishmen to attempt it, and Byron made notes for Military Intelligence—is still hard and perilous today. Travel, he wrote to his mother, was “a grindstone to temper one’s character and get free of the cloying thoughts of Europe.”

If there is a defect in The Road to Oxiana it is that of Byron’s time and class. Despite his respect for Persian art and his disdain for British convention, an ingrained condescension pervades his relationships with the modern inhabitants. He rarely attempts empathy, and at worst he is downright choleric. Several times he physically assaults people who infuriate him. And the diaries from which his book was refined are, if anything, more intemperate still. “My point of view,” he wrote to his mother, “is fundamentally that of the artist rather than the district nurse.” His descriptions, like those of Kinglake, are peppered with Anglocentric references. (A tweedy road supervisor looks like Lloyd George; the Murghab river is “about the size of the Thames at Windsor”, and an asphalt road down the Khyber Pass is as smooth as Piccadilly.)

But Byron’s overweening character and glittering mosaic of a book were all of a piece. And its final excerpt brings him home to his beloved mother. “What I have seen she taught me to see,” he writes, “and will tell me if I have honoured it.” This was no idle accolade. After her son’s death she said: “I was always surprised when he struck out or changed any words I didn’t like—immediately—I was a very hard critic.…”

Almost a year and a half went by—months consumed by lecturing, articles and the transposing of his first two travel diaries—before Byron joined Desmond Parsons, who was living in Peking, and began again on The Road to Oxiana. But this most celebrated of twentieth-century travel books was restarted in profound dejection. Parsons was diagnosed with the fatal Hodgkin’s disease, and returned to London for treatment. Byron remained through a bitter winter, struggling to write. He was struck down by fever, then neuralgia. He worried about money. He started to drink, and to veer out of control. At one dinner he smashed the china and glass at the British Embassy. “My muse is dead,” he wrote to his sister.

But it was only sleeping. He settled at last to a discipline of six hours a day, interspersed with herbal tea. Audaciously he kept the narrative form of his diaries intact in all their freshness and panache, only pruning a little here, easing the narrative flow there, and buttressed them with retrospective research. At the end he wrote to his publisher, Harold Macmillan, that the book surprised him by its substance: “I venture to think it is the best thing I have written.”

It is hard, in so individual a work, to trace what others may have influenced it. Earlier travellers to Persia, like Lord Curzon, leave no perceptible trace, and the travel books of Byron’s contemporaries, notably Peter Fleming, proceed in a time-honoured linear narrative. Byron admired Norman Douglas, whose works were as erudite and richly styled as his own, but The Road to Oxiana, in its fluent demotic voice, has survived more surely into the twenty-first century than Douglas’s Old Calabria.

Byron’s book was published in 1937, to varying acclaim. Graham Greene found it alternately brilliant, gossipy and “dryly instructive”. Evelyn Waugh crossly accused it of self-centredness, yet praised its gusto and dialogue; and G.