Shadow of the Silk Road, by Colin Thubron

I could worship Colin Thubron. Time and again he has reminded me what a good book is. Too often he has been typecast as a travel writer; there is a clutch of underrated novels, too. But if travel-writing is his forte, he brings to that genre an extraordinary range of skills, not least physical endurance in difficult, dangerous terrains. His descriptive acumen is boundless, and he is never anything but well-prepared. He learns the history before he sets out, and enriches it through his observations. His journeys are leavened by the true adventurer's susceptibility to the unscripted. He acquires languages, and has a gift for turning chance encounters into cultural excavations. A "travel book" by Thubron is always literature, often of the highest calibre.

Shadow of the Silk Road recharges my admiration. Here Thubron undertakes the most arduous journey of all, across the faded belt-line of the Eurasian landmass, thousands of miles of it. Only once does he resort to a quick plane-hop, between Mazar-e Sharif and Herat in still war-torn Afghanistan. For the rest, he chooses the least comfortable transport: hitched truck-rides, clumsy buses, third-class carriages. Camels, horses and ponies are familiar to him; one yak as well.

When the only way to reach an Assassin cave-fortress is to scale a sheer rock-face, Thubron scales it. And he is in his sixties when he does all this. Mad, of course. Obsessed - but obsessed with what? Destinations constantly loom ahead, but reaching them promises little satisfaction. It's the old saw about the journeying meaning more. He travels light: a rucksack full of maps, primers, notebooks, money secreted in an anti-mosquito spray.

There is very little by way of clothing, even less of ego - but there is a phantom alter ego that tags along, a Sogdian trader who quizzes Thubron about his motives. From their imagined dialogues it emerges that Thubron's obsession is chiefly with death. Or is it that too much of the decayed landscapes he traverses has rubbed off on this leathery, erudite explorer, enamoured of legend and fable?

Fable thrives in lonely, loveless places. Thubron begins in the heart of old China, at Huangling, site of the supposed tomb of the mythical Yellow Emperor. He progresses to bustling Xian, once Chang-an, capital of the founding Qin dynasty. The modern Chinese he meets are secularists, their dreams borrowed from America. But entering the Gansu corridor, all that evaporates. The way ahead is strewn with destroyed cities, ruined monuments, empty sepulchres: the shell of Samarkand.

In defiance of the map's frontiers, peoples, cultures and faiths only slowly merge into one another. Westwards through the awesomely barren Tarim Basin, he traces a once buoyant Buddhism. Well before he reaches Kashgar, shaded by the Pamir mountains, Islam rises with the tyrannised Uighurs. In Kyrgyzstan, then Uzbekistan, an emasculated Islam greets him - suborned by the departed Soviet Union's secularism.

So much of what Wintle half-finds in his journey's middle reaches was destroyed by the Mongols. Yet Thubron refers to "the Mongol Peace": they crushed the Assassins, the al-Qa'ida of those days, to the glee of Christendom. So he journeys onward through northern Iran (a fetid police state as much as a theocracy) and southern Turkey (the Kurdish question there) until he reaches Antioch, once an eastern outpost of ancient Greece, thereafter home to an incipient Christianity.

Thubron takes it all in. His witnesses are mainly losers: victimised professionals, underground artists, oppressed women. These are the people who needs must speak to him. Only Ismail Khan, Shi'ite leader and former main player in Afghanistan's internecine woes, is a known quantity. But Khan, too, is diminished: a superannuated warlord. For once, no illuminating conversation flows. So it is back to Thubron's phantasmic Sogdian trader. Between them they poke away at individual faiths, at faith itself.

Centuries before the opening of the Suez Canal in 1869, the globalising Silk Road was irreparably frayed. Quest and pessimism entwine, taking me back to that other monumental 12-chapter epic, Levi-Strauss's Tristes Tropiques, which Thubron's magisterial threnody matches.

Justin Wintle's biography of Aung San Suu Kyi appears next year