www.guardian.co.uk Colin Thubron's latest book, In the Shadow of the Silk Route  comes with some anticipation. The master of Eurasian travel has exploited the mutability of the Silk Road to guide him through lands he has visited over four decades. Unfortunately, his desire to journey its length in one epic trip was disrupted by Sars in China and warlords in Afghanistan. This perhaps is appropriate; plagues and politics have interrupted many before. But it means that Thubron's journey was split across two years, even though we hardly notice the join.

What is noticeable - and unexpected - is the unease. Thubron starts in China, admitting that he has not lost his "misgiving at this harsh land". If he was hoping to understand it better on this visit he fails, although he gets so close. As he travels out of China, he notes that "everywhere along the Silk Road genetic confusion reigns" and he describes characters "living in a world of selective truth". But he tantalisingly fails to subject China itself to the same scrutiny as other Silk Road countries or to recognise that it too is a complex mix of ethnicities and its historical narrative, although very convincing, is just as selective. The primary cause of his misgivings seems to be the cultural revolution; a terrible time, but equalled by horrors in other countries whose peoples he forgives more readily.

In this first section he also recounts elements of the seductive modern narrative of the Silk Road - Chinese silk found in Egyptian tombs; a Christian church in China; descendants of a Roman legion in a desert village; and mummies dressed in tartan. Some of these, too, are myths, or at best selective truths. "You would go mad," he says, "tracing the ancestry of the simplest things", and perhaps it is too much to expect him to have seen through the myths that continue to entrance so many.

Thubron has the seeming ability to charm women and yet be one of the boys. In Behind the Wall, which describes his journey from Beijing to Tibet, there is a surreal incident where he is entertained to a private fashion parade in a remote Chinese house. In each of his books there is at least one scene of drinking hard liquor late into the night with dubious companions. We rely on travel writers for their openness to tall tales and for their willingness to seek encounters with those the rest of us would make every effort to avoid. This, of course, makes Thubron's story as much a selective truth as a history of the Silk Road. And he never wants for bizarre characters, among them the would-be Chinese entrepreneur imagining riches in Brazil and the Iranian student caught between delight and revulsion at Jennifer Lopez's bottom. Thubron lets them speak, but does not disguise his irritability at their persistence or his anger at their prejudices.

Fortunately he loosens up as he leaves China and enters a land where he can speak Russian, whose "soft consonants ... came more easily to me than a half-forgotten Mandarin". He chooses a route through Kyrgyzstan and Uzbekistan - described in the infancy of their independence in The Lost Heart of Asia. He depicts countries struggling to find a post-Soviet identity in their Silk Road past. He is then lured, somewhat dangerously, into northern Afghanistan.

Travel writers fall into two categories: those who play the hero - the Sven Hedin model - and those for whom their own role is always secondary to the interests of the journey, such as Aurel Stein. Thubron is in the Stein mould. In previous books we have been lulled by his seeming insouciance, only occasionally stopping to wonder at the dangers. Like a character in a fairytale, he tells us about the trolls on the bridge, but we never really believe they will reach him. Here, possibly under pressure from his editor or possibly owing to a preoccupation with his own mortality, the danger is given greater weight. At one point, he is almost killed when the man he has been drinking with drives their car straight towards an oncoming lorry. Miraculously they survive. On another occasion he scales a cliff, almost freezing 30 feet up, and unnecessarily spelling out the danger to the reader.

His own mortality is also prominent in some whimsical dialogues with an imaginary Silk Road character: a Sogdian merchant. These are uncharacteristically mawkish, puncturing rather than punctuating the narrative. This is a shame, as Thubron's justly renowned prose style is evident elsewhere, especially in throwaway descriptions of Bactrian camels as "huge moulted beasts with lax lumps and chewed ears" or Buddhist monks at the Dunhuang cliff caves who "painted its darkness with their faith".

Ironically, it is when he crosses to the Persian-speaking section of the Silk Road that he seems finally to relax, even though here he has no language skills. We certainly race through the Iranian world. History seems more distant and the Sogdian merchant makes fewer appearances (fittingly, as Parthians tried to prevent the Sogdians from trading here). Perhaps this is because he is on the home leg, or maybe it is simply that much of this land is new to Thubron.

Although not vintage, this is certainly a matured Thubron. But maturity does not necessarily lead to mellowness: in some cases it can leave a bitter aftertaste, requiring effort to understand its complexity and complexes. Thubron delivers some wonderful nuggets on the Silk Road, but one is left with the feeling that he might have been better searching out new lands rather than revisiting old ones.

Review by Susan Whitfield.