Few authors have left such an immediate legacy as Roger Deakin. When he died of a sudden illness in 2007, aged 63, he had written just two books: Waterlog, which set off the wild swimming craze, and the even more influential Wildwood, which helped kickstart the publishing phenomenon of nature writing. Yet both books only really became well known after his death. During his lifetime he was, at best, a cult taste. When I approached the BBC 20 years ago with the idea that he should present a televisual version of Waterlog in which he swam ‘across’ England, through its ponds, lakes and rivers, I was told no one was interested in wild swimming – and who was Roger Deakin anyway?

Patrick Barkham has set out to answer that question. The results are unexpected even for those of us who knew Deakin. A man of many lives and compartments (and relationships), he himself thought that what you needed to write was ‘energy, sexual potency and solitude’. Having come of age in the 1960s, he brought a maverick and restless intelligence to his writing, which began late after an equally restless career in advertising, furniture restoration and as a campaigner for ecology. He helped found Common Ground, which pushed, among other things, for more variety in apples against the super-market monoculture.

When he came to write what he first dubbed ‘the swimming book’, he was living in Suffolk, at Walnut Tree Farm, in a cottage with a moat which became essential to his writing. It was where he began his swim across England. ‘Moat’ makes it sound as though he was living in a castle, but actually it was a farm cutting in the scraggy badlands of inland Suffolk around Eye, attached to a modest home. When I visited with my young children, they were entranced as Roger first showed them the many and yet-to-be fashionable shepherds’ huts he had collected. Each had a bed and sheets, so every night he could choose where to sleep, like a wanderer, and there was an outdoor bathtub in the meadow. But they were disappointed later as he revealed – always the showman – he had a conventional house hidden away as well. When he swam in the moat (‘more like a linear pond really’), he emerged dripping in weed.

Waterlog became a clarion call to reclaim wild swimming – there are enjoyable brushes with authority for daring to swim in their water, including the pompous wardens of Winchester College – and a more spiritual quest to ‘leave his baggage behind and float free’. It struck a chord that resonated with many. But his masterpiece is Wildwood, which also retains an elegiac quality, since he was still trying to complete it on his death and it was published posthumously. Indeed, he was wrestling with the idea that it was two books joined at the hinge: one on the woodlands of Britain, the other ranging far further afield, to places such as Kyrgyzstan.

It begins beautifully, in the kitchen of the ash-framed house he had largely built himself. One can almost hear his rich voice:

In the pine top of my work table, the dark knots are boulders standing up in the rivers of grain, sending eddies and ripples spinning downstream, delivering the driftwood thought of a new journey to be taken, through trees.

Deakin revived the literary tradition of Gilbert White and Geoffrey Grigson, not only in the rigour of his style (those days as a copywriter had not been misspent), but in the way he was comfortable with the interweaving of nature with art, history and a life lived outside the narrow confines of biology. He was at ease with the countryside as only a poacher with very deep pockets can be.

The book was completed by Robert Macfarlane, who deserves great credit for honouring Deakin’s legacy and carrying his literary torch. Now Barkham, the nature-writing correspondent of the Guardian, has written this fine tribute. He has idiosyncratic form as an author, which makes him a good fit for Deakin. When promoting his earlier book, Badgerlands, he toured the country wearing a badger onesie, which made him stand out at literary festivals.

In the introduction he explains that he originally set out to write a conventional biography of Deakin but, having completed it, decided it would be better to have more of Roger’s own words. So he writes sections as if he were Roger, ventriloquising him in the first person, interspersing diaries and letters with his own additions, and speaking as his ‘ghost’. The result, while a bold strategy, is only partially satisfactory – we wonder at times whose voice we are hearing – and perhaps there should have been two different books. But he does explore Deakin’s complicated love life through candid interviews with many former partners. Roger was certainly, as one friend says, ‘not a green saint, and far more interesting’. The picture that emerges is of a man who was driven, and difficult to live with, yet whose core response to the call of the wild animated everything he did.

The sections on his death are almost too painful to read. There is some comfort in knowing that he had just heard of the research into how trees are interconnected by their roots and share nutrients via fungi to help each other when sickening – science confirming what he had always sensed instinctively.

There was a pollarded hornbeam near his home, shaped like a church bell, into which he would swing himself to read, and where all he could hear was ‘the wind making a sound with the quality of a shingle seashore not far away’. Barkham imagines Deakin’s reaction of ‘shock and outrage’ at this book, but he is too hard on himself. It is an honest tribute to a towering figure who died just as he was in full flow.