IF Colonel Gardner had not existed, it would not have been possible to invent him. He did a fairly good job of that himself, and where there were gaps, they were imaginatively filled by later biographers or crossly left blank by debunkers, who tended to think the whole thing was a clever sham. That he existed is not in doubt.

For the magnificent photograph of “Gordana Khan” on the cover of his book, he stiffly posed in trews and turban, which was feathered with an egret plume to indicate a man of great distinction. The tartan appears to be that of the 79th Highland Infantry, to which he had no claim or right. He was 79 when the picture was taken and still had another few improbable years left in him.

Who he really was is still a matter for debate. The most convincing version is that Alexander Haughton Campbell Gardner was born in Wisconsin in 1785 to a Scots father and English-Spanish mother. At least one writer who met him believed him to be Irish instead, perhaps because Gardner spent some time there in his mid-20s. He made a brief return to America but with his father dead and his future there apparently foreclosed he returned to Europe and headed East, first to Astrakhan, and then on a series of journeys into unknown places and ambiguous identities. The first, in 1819-21, was a modest loop round the fringes of Persia, Turkmenistan and Afghanistan. The second, a couple of years later, fringed the Alai Mountains and down to Kandahar. The last and most spectacular strayed into China over the Kongur Shan, through the Pamirs, and into untrodden Kafiristan. If only a tenth of it were achieved, it would still be spectacular. How did he survive? History doesn’t reserve much admiration for mercenaries, but that is what Gardner became, offering his services first to the Russian army, later to whatever adventurer of Sikh maharaja would have him. He claimed to be a Muslim; he claimed to have had an Eastern wife and child, who were murdered. His incomplete journal claimed lots of things and was eventually (after his death in 1877) edited into a reasonably coherent narrative by Major Hugh Pearse.

Subsequent historians chose to disbelieve much of what Gardner claimed, but there is enough to support at least some of his more elaborate deeds, including a heroic role in the relief of Lahore in 1841 when Gardner seems to have single-handedly fired the guns that destroyed the attacking force. What the famous photograph doesn’t show, and the great beard helps to conceal, is a body criss-crossed with wound-scars, including a slash across the throat that required him to use a clamp in order to swallow food and wine.

The sceptics who have difficulty swallowing anything to do with Gardner are sometimes insufficiently aware that as a figure who has drifted into legend, he exists as much as text as he does as man and soldier. He plays a key role in George MacDonald Fraser’s Flashman and the Mountain of Light. To some, he seems to have taken his tales of wandering in the Pamirs and Kafiristan from Rudyard Kipling’s The Man Who Would Be King, when in fact Kipling took his story from Gardner’s. What exists of Gardner is what was written by him and about him. Even the daughter who followed his trail years later, in hope of recovering an inheritance thought to be worth millions, had to contend with rumour and report and not much in the way of notarised fact. She is, perhaps, the most poignant figure in the whole book.

Whether the focus is long and synoptic or short and detailed, Keay is always a fascinating guide to India and the origins of the Great Game. Here, the focus is often very precise indeed. The downfall of the Sikh empire, in which Gardner played an important and often morally questionable role, is set out with great precision. Gardner’s life is wisely regarded as a pattern of truth, half-truth and invention or simple forgetfulness. The older Gardner was pressed for stories and he gave them up willingly, though not always consistently.

It is perhaps most rewarding to read The Tartan Turban as if it were a novel by Thomas Pynchon, full of odd collisions between history and fantasy, modernity and ancient tradition, intriguingly paranoid in its sense of historical flow.

Keay’s mastery of the subject – which he began to explore 40 years ago in When Men and Mountains Meet – put him very naturally at the head of a new list from Kashi House, an imprint dedicated to Indian culture, Sikh history and the Punjab. Very handsome it is, too.