Travel writing  an individual telling a story about a journey through a landscape  is one of the world's most primal forms of literature. Tales of travel take us back to man's deepest literary roots, to the Epic of Gilgamesh and the wanderings of the Pandava brothers in the Mahabharata. Like epic poetry, but unlike the novel, the travel book has appeared spontaneously in almost all the world's classical and medieval cultures, from the journeys of Hsuan Tsang in India and Basho in Japan, through the topographies of Ibn Battuta and Marco Polo, to the Celtic monks venturing westwards on their immram wonder-journeys.

Recently it has become almost a cliche to predict the extinction of travel writing in the age of the internet and Google Earth. And yet this ancient form stubbornly refuses to die. For every piece predicting its demise there is another announcing the arrival of some new talent: in the past few years, writers as diverse as Pankaj Mishra, William Fiennes, Suketu Mehta, Rory Stewart and Peter Hessler have all produced masterworks that show the continuing vitality of the travel book, as well as its ability to reinvent itself for each successive generation. But of all these, there is one in particular who has shown how utterly beautiful a brilliantly written travel book can still be. That writer is Robert Macfarlane.

Macfarlane, a young English don at Cambridge, produced his first book in 2003: Mountains of the Mind was a genre-defying look at man's fixation with mountains. It won immediate acclaim and a cabinetful of awards. But it was Macfarlane's second, the rich and lyrical The Wild Places, that first showed how far he was capable of out-writing almost any other prose stylist of his generation.

The Wild Places is, as its title suggests, a search for the feral in modern Britain; but it as much an exploration of the interior of Macfarlane's mind as it is of the wilder reaches of the British landscape. The tangled path it weaves through history and memory, literature and landscape, high-flown prose and earthy observation rises to a bravura climax in its final chapter. Here Macfarlane concludes that "the wild prefaced us, and it will outlive us. Human cultures will pass, given time, of which there is sufficiency. The ivy will snake and unrig our flats and terraces, as it scattered the Roman villas. The sand will drift into our business parks, as it drifted into the brochs of the iron age. Our roads will lapse into the land."

Now, after five years of work, Macfarlane has produced a sort-of sequel. The Old Ways is in some ways a continuation of its predecessors, being also about the connections between man and landscape. While in The Wild Places the chapters are arranged by topography  Beechwood, Island, Valley, Saltmarsh and Tor  in The Old Ways we have geological textures: Chalk, Silt, Peat, Roots and Flint. In other ways, however, Macfarlane inverts the concluding proposition of The Wild Places. For in The Old Ways the roads are shown to be almost indestructible, as if existing in geological rather than in human time, binding man to his past.

Macfarlane's search in this book is for the ancient routes that criss-cross the landscape  mainly in Britain, but with occasional forays to more exotic spots. So we meander with Macfarlane not just along the old tracks of the Icknield Way and the Ridgeway, but also, more fleetingly, on "a branch line of the most famous pilgrimage route of them all, the Camino de Santiago" and on Buddhist trails in the eastern Himalayas, exploring the links between topography and belief. The subtitle of the book is "A Journey on Foot", but in reality it is not one journey, but many, and not all are on foot: some of the best passages are about the old seapaths and ocean roads linking the islands of the Outer Hebrides with Norway, Iceland and Orkney. Like the pathways that weave the countryside together, there is no central spine to this book. Instead it is held together by a tight matrix of ideas about "the compact between walking and writing", and how roads bind us to the land, and to our past.

The poet and walker Edward Thomas (1878-1917) is a constant presence. It was his book on the Icknield Way that first led Macfarlane to his theme, and Macfarlane is fascinated by Thomas's idea of how an ancient road can be part of a ghost world "secretly sharing the landscape with the living" where you can connect with the thoughts, feelings and stories of previous walkers along the same footpaths: "walking as seance". He writes how "in the dusk of the Holloways, these pasts felt excitingly alive and co-existent  as if time had somehow pleated back on itself". Like Thomas, he is in love with the notion "that history issues from geography in the same way that water issues from a spring".

Bruce Chatwin is another clear influence and, like Chatwin, Macfarlane believes that walking is both therapeutic and inspirational. Yet his recklessly poetic and sometimes almost mystical speculations are always firmly rooted in the precision of his observation and reporting and irrigated by the wide variety of different interests he brings to his books. As an English don, he is profoundly literate, and here he brings the full weight of his erudition to the table. In different places in this book he quotes a dazzling range of obscure poets and novelists as well as great galaxies of writers on walking, wildlife and landscape.

Macfarlane can also tell a good story, and is companionable and funny: unlike many nature writers, he likes people, and his landscapes are filled not just with animals, stones and plants, but the countrymen  sailors, botanists, poets, archaeologists and crofters  who inhabit these remote places. Some he seeks out for their knowledge, others for their stories. One he admires for "his ability to read landscapes back into being, and to hold multiple eras of history in plain sight".

Above all, perhaps, Macfarlane brings to his books his love and knowledge of the natural world, and so cross-fertilises the rich till of his travel writing with the loam of another very English tradition of observational literature: nature writing. He knows his plants and his flowers and can tell the "screech of a barn owl" from "the furry hoot of a tawny". He is poetic and lyrical in his approach to the natural world, but can also be precise and scientific: while in The Wild Places we were given the best one paragraph explanation I have read of why leaves turn brilliant colours in the autumn, here there is a lucid description of why the eyes of certain animals reflect light in the dark  with each species giving off a slightly different shift in colour.

With this mastery of both travel and nature writing he brings together into confluence two great streams of British nonfiction. There are echoes here of Roger Deakin, Ted Hughes and WG Sebald, and, more faintly, of their American counterparts, Peter Matthiessen and Barry Lopez. But Macfarlane seems to have learned especially from the careful observation and incandescent prose of one of his heroes, JA Baker, the anonymous Essex librarian who wrote one of the great classics of 20th-century nature writing, The Peregrine, a book that Macfarlane has championed and for whose US edition he wrote a fine introduction.

Like Baker, Macfarlane is read above all for the beauty of his prose and his wonderfully innovative and inventive way with language. Like Baker's peregrines, he stoops with unerring accuracy on his prey  the perfect image, the most elusive metaphor  and he can write exquisitely about anywhere, even Royston. This book is as perfect as his now classic Wild Places. Maybe it is even better than that. Either way, in Macfarlane, British travel writing has a formidable new champion.

William Dalrymple's new book, Return of a King: The Battle for Afghanistan, will be published by Bloomsbury in February 2013

"A Journey on Foot", reads the subtitle, but this is the story of many journeys. Fifteen of them are made by Macfarlane himself, along paths in the British Isles and, further afield, in Spain, Palestine and Tibet. He invokes, as he goes, hundreds of previous walkers, and hundreds of pathways  across silt, sand, granite, water, snow  each with its different rhythms and secrets. So the book is a tribute to the variety and complexity of the "old ways" that are often now forgotten as we go past in the car, but which were marked out by the footfall of generations. And it is an affirmation of their connectedness as part of a great network linking ways and wayfarers of every sort. Following Macfarlane's many travels, one understands why he thinks of his project as "a journey", singular rather than plural. In this intricate, sensuous, haunted book, each journey is part of other journeys and there are no clear divisions to be made.

"Leys, dykes, drongs, sarns, snickets  bostles, shuts, driftways, lichways, ridings " Macfarlane will have many of his readers dreaming in path-language this summer. Certain images keep glimmering in the dark when I close my eyes: fragments of white china clay scattered as a trail across the bogs of Dartmoor; marker stones on Bodmin guiding a parson safely around his parish; posts sticking up from the water in the monochrome mirror-world of flooded Doggerland where the narrow "Broomway" leads out to Foulness.

Macfarlane's first two books, Mountains of the Mind (2003) and The Wild Places (2007), were published to huge acclaim and have achieved the status of modern classics. The Old Ways joins up with them to form what Macfarlane calls "a loose trilogy about landscape and the human heart". That definition is striking. It takes some courage for a writer to say that his subject is "the human heart". It sounds a little old-fashioned, a little out-of-step with modern detachment. But that is part of what makes Macfarlane's voice significant. He willingly declares his love of things. He brings his powerful intelligence to bear on the need to express sentiments and sensations.

He keeps asking, "what does this feel like?" Walking barefoot on Lewis: "The peat was slippery and cool, and where I stepped on sphagnum it surged up and around my foot, damp as a poultice." Or in Hampshire: "I was walking in a stormlight that made the linseed pulse a hot green  Dark shoals of rooks over the woods, and billows of rain like candle-blacking dropping into water." He is wry about his own romanticism ("what I thought was the first star turned out to be the night light for a plane coming into Luton"), but he wants to make space for it.

One of the most compelling chapters is concerned with a path across the Isle of Lewis to shielings, or stone shelters, built by crofters near their summer grazing grounds. The path is detectable only by learning how to read the stony landscape. "Look for what shouldn't be there," Macfarlane is told, so he looks for minor disturbances in the lay of the land, dots that only become visible when connected. The moment of path-finding is a revelation, analogous to the moment at the end of To the Lighthouse when Lily suddenly sees where she must paint a line on her canvas. "Click. Alightment," writes Macfarlane simply: "Blur resolving into comprehension. The pattern standing clear: a cairn sequence, subtle but evident, running up from near Dubh Loch shore."

Each chapter of The Old Ways is composed of many short passages built up like little cairns, or strewn like shards of china clay. In memory they keep forming new alignments. The making of new maps  both of narrative and of land  is one of Macfarlane's enduring fascinations. His project in The Wild Places was partly spurred by a realisation that for most of us the map of Britain is the road map. He set out to trace an archipelagic map so different from the AA atlas that it was almost beyond recognition. In The Old Ways he studies Britain geologically, exploring the relation between peat and gneiss, chalk and sand, asking how we can learn to understand the country differently.

Macfarlane is delighted to discover that the verb "to learn" links back etymologically to proto-Germanic liznojan, meaning "to follow or to find a track". The walking of paths is, to him, an education, and symbolic, too, of the very process by which we learn things: testing, wandering about a bit, hitting our stride, looking ahead and behind. That is the rhythm of learning in all kinds of disciplines and ways of life. Whether we are in the kitchen, the library or the laboratory, we are seeking out paths and deciding who to follow. So this is very much a book about learning. Macfarlane presents himself as a student in the ways of the land, taking lessons from those who have spent their lives negotiating particular kinds of path.

The deepest of the friendships written into Macfarlane's books have been with mentors. The turning point of The Wild Places was a walk taken in the company of the naturalist Roger Deakin, who pointed out that a little crack in limestone contained a wilderness: "Miniature, yes, but fabulously wild." Accordingly, Macfarlane made sure that the culmination of his book was not the ascent of a summit but the slow un-sublime observation of life in one of the sunken holloways of Dorset. The description of that burrowing walk became Macfarlane's elegy for Deakin, who died in 2006. It was the pupil's tribute to a teacher, an affirmation that something valuable had been inherited and will be passed on again.

There is another elegy in The Old Ways  for Macfarlane's grandfather. And there are many new teachers: a sailor skilled enough to cross the Minch to the Shiant Islands; a sculptor and a Tibetologist; a friend who knows the danger and importance of walking in Ramallah "discovering stories other than those of murder and hostility". They all become important figures in a book about the ways people come to know places.

There are textbooks too. A Victorian field guide, for example, describes Agrimonia in rather uncompromising terms: "Herbs with stipulate, pinnate, serrate leaves and terminal bracteate spine-like racemes of small yellow flowers." Macfarlane is not much the wiser. "I was pressed to think of a description less likely to help me identify agrimony when I saw it." He quotes that little snippet from a past age of botanical expertise as a kind of public self-reproach. A nature writer, after all, should probably know his field flora. But then again, the quotation serves to emphasise the distinctiveness of Macfarlane's nature-writing in The Old Ways. He wants to find a language for sensory experience, and to test the languages used by walkers before him.

Macfarlane's way of looking and describing is shaped by two men in particular. In one chapter he takes his bearings from the watercolourist Eric Ravilious, "a votary of whiteness and remoteness, and a visionary of the everyday". Taking to his skis somewhere north of Swindon, Macfarlane experiences the Marlborough Downs  via Ravilious  as a variation on the Arctic. Ravilious spent most of his working life not on chalk downland but in Essex; he is, to my mind, just as brilliant when painting cucumber frames in a greenhouse as when he renders the chill of ice. But Macfarlane's version of him brings out qualities I would never have seen.

The chief guiding spirit of The Old Ways is Edward Thomas, walker, nature-writer and poet, who left the "South Country" he loved and followed the chalk across the channel to northern France, where he died on the first day of the Battle of Arras. In a sense The Old Ways is an experiment in geographical biography, asking how much we can understand of another's life by inhabiting their places and following their tracks. As such it is deeply indebted to a modern tradition of biographical path-following that goes back to Richard Holmes's luminous Footsteps. And it is fascinating in being completely unlike  but complementary to  All Roads Lead to France, Matthew Hollis's beautifully observed study of Thomas's last years.

One senses Macfarlane trying to keep all his subjects in balance: he is writing about Thomas, about himself, about himself tracking Thomas, about paths in general and in particular. At times there are too many points of focus. But this is a spacious and inclusive book, which allows for many shifts in emphasis, and which, like the best paths, is always different when you go back to look at it again.

 Alexandra Harris's Virginia Woolf is published by Thames & Hudson.