The final volume of Patrick Leigh Fermor's trilogy is almost as hard to review as it was to write. By the time he died, in 2011, 96-year-old Leigh Fermor had acquired near legendary status. In the second world war, he assisted in a partisan mission to kidnap a Nazi general on Crete. Before that, at 18, he walked across Europe from the Hook of Holland to Istanbul (which he still called Constantinople). And he kept notes as he walked. "My whole life had seemed to revolve around those stiff-covered exercise books," he said. "Keeping them up to date had acquired the charm and mystery of a secret religion, solemnized daily." The books came later much later.

In 1962, a US magazine asked him to write about walking  a 5,000-word commission that spawned a trilogy. A Time of Gifts was published in 1977. Between the Woods and the Water, which appeared in 1986, ended with the promise that the story would be continued. Rumours as to whether Leigh Fermor had managed to complete his trilogy, or whether he had even started the conclusion, have circulated for the past couple of decades.

It turns out there was a manuscript and it picks up where the previous one ended  at the water, the Danube, and the Iron Gates, a gorge at the Romanian-Bulgarian border. It takes Leigh Fermor not to Istanbul, the intended final destination of what he called "the Great Trudge", but to Burgas, 50 miles from the Turkish border.

Although Leigh Fermor was still rewriting the manuscript shortly before his death, the work was problematic, for reasons Artemis Cooper makes clear in her brilliant recent biography of the man. Now she and the travel writer and novelist Colin Thubron have prepared the work for publication. This, clearly, has involved more than spell-checking, although they claim that "there is scarcely a phrase that is not his".

The first two volumes were a joy to read, not least for Leigh Fermor's ability to recapture in later life the intense excitement of being a young man lighting out. The latest book offers similar joys. His allowance of £1 a week  bank notes arriving like manna at post offices along the way  was enough to live modestly. Travelling mostly on foot, in leather jacket, knee breeches and puttees, with backpack, Hungarian walking stick and "uncompromising" boots, carrying two books of verse in the backpack and a head full of literature and history, he has his fair share of luck and adventure in a continent that was still a mystery. There are nights in shepherds' huts, down-at-heel hotels, palaces, and a brothel he mistook for an inn. And while his older self clearly enjoyed writing about the nights of revelry around campfires with belly-dancing Greek fishermen and other wild characters, he was also happy to laugh at the young Leigh Fermor  for not realising that the woman who welcomed him so warmly into the brothel expected more from him than his head on a pillow.

Also evident are another of the joys of the earlier books  the pyrotechnics of his writing. Exuberance is expressed in heightened suggestions: a cat is panther-like, a silence falls "like angels flying overhead" and swifts make a sound like scissors in a barber shop. The descriptions of waking in unfamiliar places are so seductive that even the most home-hugging reader will long to wake somewhere unknown. And some of the evocations of landscapes and views will live long in the memory, including one of a muezzin calling from a mosque and another of the town of Tirnovo, with its "winged insurrection of houses plumed by belfries and trees".

The first two books were written without the help of original notes, which had been lost; The Broken Road is based partly on a diary that was returned to Leigh Fermor in 1965. So instead of writing what the editors call "memory-spurred recreations", we see the older man trying to guess what his younger self did or why he did it. There is also retrospective comment on Europe between the wars from an author who knows that the rise of the Nazis and the coming cold war are about to transform the lives of most people he meets.

Leigh Fermor completed his physical journey in Istanbul on the last day of 1934, then continued to the monasteries of Mount Athos in Greece. The literary journey concludes without reaching its goal, hence the book's title. The editors have included sketchy diary entries for Istanbul and more fully written descriptions of Mount Athos, although Leigh Fermor was not convinced about putting them in his story, and with good reason.

The bulk of The Broken Road was written 30 years after the journey. I am reading it 50 years after it was first put down. While it is not the literary masterpiece it might have been had Leigh Fermor been able to work his magic, it captures the joy of the open road, the fresh view he gives of Europe as it began to show the stresses that led to world war, and the glimpses of a long-lost life and innocence.

After a lifetime of travel, Patrick Leigh Fermor made his last journey in June 2011. Under a cloudy, grey English sky, his bier was carried out of the church at Dumbleton, in the Cotswolds, towards the waiting grave cut.

It was in some ways almost a military funeral: the coffin was shrouded in a union flag, a military piper sounded "The Flowers of the Forest", and the wooden coffin was slowly lowered by six grey-haired veterans in uniform. Then the last post was played by a bugler in a bearskin.

Yet the readings that had preceded the burial came from a very different, more literary world. One was from the apocryphal Book of James, describing a moment when time stands still; another was an arcane passage from The Garden of Cyrus by the Restoration mystic Sir Thomas Browne: "But the Quincunx of Heaven runs low, and 'tis time to close the five ports of knowledge."

In death, as in life, Leigh Fermor was a master of bringing together different worlds that one would normally have imagined to be opposed, if not incompatible. A one-man compendium of contradictions, Paddy (as he was known to everyone) was a genuine war hero. He abducted the German commandant of Crete, and in the movie of the exploit, Ill Met By Moonlight, Paddy was played by Dirk Bogarde. Yet this man of action, with the speech patterns, polished brogues and perfect manners of a prewar British major, was also one of the great masters of English prose.

Paddy was equally at home with both high and low living. His masterpiece, A Time of Gifts, the first volume of a trilogy, tells of his walk from the Hook of Holland to Constantinople in the last days of prewar Europe, "like a tramp, a pilgrim, or a wandering scholar", as he moved from dosshouses to Danubian ducal fortresses: "There is much to recommend moving straight from straw to a four-poster," he wrote, "and then back again."

On the wet afternoon of 9 December 1933, the year Hitler came to power, as "a thousand glistening umbrellas were tilted over a thousand bowler hats", Paddy left London, boarding a Dutch steamer at Irongate wharf. His rucksack contained pencils, notebooks, The Oxford Book of English Verse and a volume of Horace. He would not see Britain again until January 1937, when he returned "for better or for worse, utterly changed by my travels".

"I thought I'd keep a diary and turn it into a book," he told me when I went to stay with him at his house at Kardamyli, in the Peloponnese, a year before he died. "Except I am still writing that book more than 70 years later. It broadened my mind, taught me history, literature and languages. It opened everything up: the world, civilisation and Europe. It gave me a capacity for solitude and a sense of purpose. It taught me to read and to look at things. I didn't go to university; I went travelling instead."

On and off, for nearly 70 years, between other projects, Paddy worked on the trilogy. The first volume, A Time of Gifts, was finally published in 1977. For many of us, his descriptions of walking through midwinter Germany in the 1930s have the status of sacred texts: "Sometimes the landscapes move further back in time," he writes in the unforgettable Winterreise chapter. "Pictures from illuminated manuscripts take shape; they become scenes which old breviaries and Books of Hours enclosed in the O of Orate, fratres. The snow falls; it is Carolingian weather."

The second volume of the trilogy, Between the Woods and the Water, followed nine years after the first. There then followed 25 years with no sign of volume three, the book that should have taken us to the gates of Byzantium. This was what his brilliant biographer, Artemis Cooper, has called Paddy's "ice age": the writer, once capable of great waterfalls of words, seemed suddenly frozen, blocked, unable to produce a page of writing that he did not later tear up in despair. There was a thin volume of letters written from a trip to the Andes, and a correspondence with the last of the Mitfords, Deborah Devonshire, but little of any substance after the 80s.

So it seemed on that June day, as we buried him, as if Paddy had left the great trilogy incomplete. Yet there were a few hints: on my last visit, he had talked enthusiastically of finally learning to type at the age of 93  previously he had laboriously written everything longhand; and there, sitting on a low table when he showed me his study, lay an 8in-high pile of manuscript, some of it ring bound and some in folders, on which was scribbled, in red felt-tip pen: Vol 3.

Then last summer, at the Hay festival, in conversation with Leigh Fermor's great successor Robert Macfarlane, Cooper announced that the long-awaited volume three was ready for the press, and would be published as The Broken Road. For 18 months, she and Paddy's literary executor, Colin Thubron, had quietly worked away editing the different drafts of the Paddy's unfinished work. Most derived from an early manuscript that pre-dated A Time of Gifts by a decade, entitled A Youthful Journey, or Parallax, which had started out as a commission from an American travel magazine on the joys of walking.

Paddy was meant to have delivered 4,000 words. Holed up on the island of Euboea in 1963, he instead found words flowing at such a rate that by the end of the summer he had written 60,000. These, characteristically, he then abandoned and threw into a bottom drawer.

The other major source for The Broken Road was the last surviving volume of Paddy's original travel jottings, known as The Green Diary. The book had been preserved by Paddy's prewar lover Balasha Cantacuzène, and was included in the small suitcase she was allowed to pack when thrown out of her estates by communist commissars at the end of the second world war. When the iron curtain began to lift, in 1965, Paddy went to visit her, and she handed over to him the only surviving original record of the journey. These jottings, dating from when he was barely 20 years old, form the concluding section of the book, and take us from Constantinople to Mount Athos.

Given how the shortcomings of this book so tormented Paddy's last decades, few of us thought it likely that it would contain any material to equal its great predecessors. The wonderful surprise is that, while the book is certainly uneven, and contains some jottings and lists that are little more than raw, unworked data, overall it is every bit as masterly as Between the Woods and the Water, while some passages  such as his marvellous account of a love affair in the old Bulgarian city of Plovdiv  are the match for some of the great passages of A Time of Gifts.

All the old enthusiasms are there: the love of language and languages, the long historical and literary digressions, the perfect recollection of a lost, Ruritanian prewar and mostly pre-industrial world. Here, too, is the same exquisite prose and the marvellous images: dead trees are "bleached white by the sun like the dismembered bones of prehistoric beasts"; the onion domes of a Russian church are "covered with a glittering and fish-like reticulation of green and gold scales"; a drunken dance by soldiers at a Bulgarian inn is "a tangle of abandoned sabres; their spurred boots were crossing and stamping with the rest, grinding the fragments of glass to smaller fragments as the dance revolved."

There are differences between this and the other volumes of the trilogy. There is more honesty about the highs of Paddy's love life and the lows of his depressions. There is less restraint about his prejudices: Turks are sadistic ruffians; Bulgars are mean-minded crooks; the Greeks, however, are heroic and romantic rustics.

There is also a greater contrast between the youthful enthusiasm of the young footsore walker and the melancholy reflections of the older man writing at home in his study. Yet by any standards, this is a major work. It confirms that Leigh Fermor was, along with Robert Byron, the greatest travel writer of his generation, and this final volume assures the place of the trilogy as one of the masterpieces of the genre, indeed one of the masterworks of postwar English non-fiction.

 William Dalrymple is the author, most recently, of Return of a King (Bloomsbury), which has been longlisted for this year's Samuel Johnson Prize for Non-Fiction.