In 1968, Roger Deakin bought the ruined remains of an Elizabethan house, and twelve acres of surrounding meadow, on the edge of Mellis Common in Suffolk. Little survived of the original sixteenth-century dwelling except its spring-fed moat, overhung by hazels, and its vast inglenook fireplace. So Roger put a sleeping-bag down in the fireplace, and lived there while he rebuilt the house around himself.

Walnut Tree Farm, the house he eventually completed, and in which he died in August, 2006, is made largely of wood. It is as close to a living thing as a building can be. When big easterlies blow, its timbers creak and groan ‘like a ship in a storm’, as Roger put it, ‘or a whale on the move’. He kept the doors and the windows open, in order to let air and animals circulate. Leaves gusted in through one door and out of another. It was a house which breathed. Spiders slung swags and trusses of silk in every corner. Swallows flew to and from their nest in the main chimney. As I sat with Roger, ten days before his death, a brown cricket with long spindly antennae clicked along the edge of an old biscuit tin.

The fields, well tended but unfarmed, were also busy with life. Sparrow-hawks busked for custom overhead, deer picked their way through the hornbeam wood and tawny owls hooted from big ash trees. The land was separated into fields by a mile of massive old hedgerow, in places five metres high and five wide. Roger had a habit of driving his cars until they were about to give out, then backing them into a particularly deep area of hedge and leaving them there, to be grown through by the briars and nested in by birds. Walking the fields with him, you would come across old Citroëns with their frogeye headlights, peeping from the brambles. ‘All that needs is a new engine, and we could drive it to France,’ he would say, hopefully, as we passed one of these.

Roger wrote as idiosyncratically as he did everything. Thinking my way through his house now, I can count at least five different desks, between which he would migrate according to his different moods. His sleeping-places changed, too. Over the years he had established in his meadows a variety of outlying structures, including two shepherd’s huts, an old wooden caravan with a cracked window and a railway wagon that he had painted Pullman-purple. He once emailed me happily about having been out in the wagon with the rain whacking on the roof. ‘An amazing thunderstorm last night as I lay listening. Like being inside a kettledrum with a whole symphony going on out there and with thunder in wraparound quadraphonic!’ When he wasn’t writing, he was usually swimming, most often in his moat. Swimming helped him think, in all sorts of ways. ‘This weather’s agony,’ he e-mailed me three springs ago. ‘It makes me want to write and write, but it also makes me want to GET OUT THERE. The moat is bloody cold, but a good solution because then I can’t wait to GET BACK INSIDE IN THE WARM.’

In his relaxed contrarianism, his environmentalism (he was a founder member of Friends of the Earth, and co-founded Common Ground, the organization which has campaigned so significantly for ‘local distinctiveness’) and his enthusiasm, Deakin was a latter-day Thoreau. Except that where Thoreau lived by his pond for a total of several months over several years, Deakin lived by his moat for nearly four decades, watching and noting the habits of the trees, creatures, wind, sun and water around him. Walnut Tree Farm was a settlement in three senses: a habitation, an agreement with the land, and a slow subsidence into intimacy with a chosen place. It was while doing lengths in his moat that Deakin had the idea for what would become Waterlog. Published in 1999 in a small print run, the book quickly became a word-of-mouth bestseller. Starting from the moat, Deakin set out to swim through the rivers, lakes, streams and seas of Britain, and thus to acquire what he called ‘a frog’s-eye view’ of the country. The result was a masterpiece: a funny, lyrical, wise travelogue which was at once a defence of the wild water that was left and an elegy for that which had gone.

You finish reading Waterlog invigorated, and with a changed relationship to water and to nature. It is a book, as Heathcote Williams nicely punned, which leaves you with a spring in your step. I have often thought that a better indicator of a book’s worth than its sales figures must be the number of letters that the author receives from readers. Roger got nearly seven hundred: he kept them all, and replied to each one on a handmade card.

The influence of Waterlog was immense. Despite its thoroughgoing Englishness, it won admirers in Australia, Canada and Europe. It prompted a revival of the lido culture in Britain, and even the founding of a wild-swimming company (a commercialisation of which Roger quietly disapproved). It also inspired untold numbers of readers to take to the open water.

So it was that, for instance, on a cold grey April day in Sutherland in 2004, I was to be found in the sprawling and remote Loch Sionascaig, in the shadow of Suilven, back-stroking out to an island while the rain fell hard on my face, already looking forward to telling Rog about the swim. The loch was a mile or so from the road, and the pleasure came at a price: I returned to my car peppered with midge and tick bites. As I reached the road, another car came into view. Its driver stopped and wound down her window. ‘You’ve been swimming,’ she said. Dripping wet, and standing in my trunks, I could not deny it. ‘A bit early in the year, isn’t it?’ she said. The midges were discouraging longhand explanations, so I said that a friend of mine had written a book called Waterlog about wild swimming, and now I couldn’t keep out of the water. She gave a surprised smile, reached down, and picked up the audio-tape of Roger’s book, to which she had been listening as she drove that lonely road.

Travel with Roger was even more unpredictable than travel under his influence. The dark-green Audi in which he journeyed to his last escapades had moss growing in its foot-wells (‘three different sorts’, he pointed out, proudly), and a variety of useful knives in the glove-box. Its boot always held a bivouac bag, a trenching tool of some sort and a towel and trunks, in case he passed somewhere interesting to sleep, dig, or swim. When lost while driving, which was most of the time, he had a habit of slowing almost to a halt on roundabouts and squinting up at the road-signs while I assumed the crash position. He was always proposing adventures: a night stake-out of a new badger warren ‘in a mysterious wooded tumulus in Thornham Woods’, or a joint attempt to traverse an acre of ancient woodland from one side to another without touching the ground, like the hero of Italo Calvino’s beautiful book, The Baron Of The Trees. ‘He’s over sixty,’ a friend said to me, ‘and he’s still got the energy of a fox-cub.’

One July we went to Dorset to explore the system of hollow-ways or ancient drove-roads which seams that soft-stone county. We ended up sleeping in a hillside meadow, and cooking in the bed of the hollow-way. ‘A Vedi shepherd in the Pindos once taught me how to make a smokeless fire’, Roger remarked idly, before creating a tiny and, yes, smokeless fire that was hot enough for us to boil water on. His extraordinary life meant that he often began stories with sentences of this kind. ‘When I was living in a cave in Southern Greece…’ ‘Did I tell you how a hunter once shot at me because he thought I was a bear?’ (The point of the story was how pleased he was to have been mistaken for an animal). We had plans to travel together to Cumbria, and at some point, Australia. He wondered if we could earn our passage out to the Antipodes as oarsmen on a quinquereme. I wasn’t sure that we could.

For the seven years after finishing Waterlog, Roger was at work on a book about woods. He disapproved of the habit of fetishising single trees – chieftain pines or king oaks. Trees to him were herd creatures, best understood when considered in their relationships with one another (he loved the way that oak trees, for instance, would share nutrients via their root systems when one of their number was under stress). Trees were human to Roger, and humans tree-like, in hundreds of complicated and deeply felt ways. Researching his book, he travelled to Kyrgyzstan, Australia, Tasmania, America, and throughout Europe and the British Isles.

Over the years the project sprawled, digressing into studies of the hula-hoop craze, Roger’s anarchist great-uncle, the architecture of pine-cones. The numbered notebooks containing his research fill a wall of the main study at Mellis. It’s now clear that a brain tumour was trying to scatter his thoughts, stop him finishing the book. But enough was done by the time he died: Wildwood: A Journey Through Trees was published in Spring 2007, and is another major work.

When my daughter Lily was born, Roger became a de facto great-uncle. For her first birthday, he gave her a tiny wooden steam engine, wrapped up in sycamore leaves. Before her first visit to Mellis, he said he had made her a present: this turned out to be a leaf-maze – thousands of bright yellow mulberry leaves that he had raked and shaped into a Lily-sized labyrinth. He showed this level of kindness and thoughtfulness to his many friends. This unstinting giving, this warmth without self-interest, drew people to him: I have never known a person so loved. It was a measure of his generosity and his devotion to nature that, even when he was ill with the cancer which killed him so fast, he could still speak unjealously of the ability of trees to heal themselves.

In early August 2006, I drove to Mellis to see Roger for the last time: held his hand, talked a little, until he fell asleep. The next day, I went with two friends, who had also known him, out to the north Norfolk coast. We swam in wild waves at dawn and dusk, and in the evening we read aloud the pages from Waterlog describing that magnificent coastline. We slept in the pine forests which run down almost to the sand at Holkham. I spent half the night in a hammock he had lent me, and half of it down on the needle carpet, where it smelt of sap and resin. Roger died a week later, still in the house that he had built around himself thirty-eight years earlier.