The last thing Katharine Norbury wanted to write was a misery memoir, although she has more than enough material for one.

There's the mystery surrounding her birth in a Liverpool convent, where she was abandoned by her mother, the miscarriage of her longed-for second child in her forties and then, within two years, being diagnosed with a rare and aggressive form of breast cancer.

Yet only about 70 of the 284 pages in her magical memoir, The Fish Ladder: A Journey Upstream, detail Norbury's personal history, which is told with admirable restraint, generosity of spirit and a total lack of self-pity.

Her enchanting book mixes lyrical writing about the natural world - an account of following watercourses from the sea to the source inspired by the great Scottish writer Neil M Gunn - and coping with grief at the loss of her unborn child. Celtic mythology, fragments of poetry and Norbury's moving search for her own source thread the book like the silvery rope of a river.

With its evocative, metaphorical title, The Fish Ladder has won exclamatory praise prior to publication: Philip Pullman ("What a delight!"), Sara Maitland ("Beautiful! Generous, moving and extraordinarily well written") and Amit Chaudhuri ("Extraordinary!"). Norbury has clearly - although unwittingly since she began writing her book more than five years ago - tapped into the current passion for the new nature-writing-cum-memoir, such as Helen Macdonald's prizewinning H Is For Hawk and Cheryl Strayed's Wild.

London-based Norbury insists that she had never planned to write anything, despite being happily married to a writer, the fine novelist Rupert Thomson, with whom she has a "sparkling" 15-year-old daughter, Evie, who has illustrated her mother's book with accomplished artwork. Additionally, Norbury was a film and script editor in television for 20 years, working with writers such as Alan Bleasdale, Harold Pinter, Anthony Minghella and Dennis Potter. "An extraordinary privilege, so it never occurred to me to write a novel," she says.

"I had nothing to say," she continues over a mug of tea in her publisher's London offices, adding that she never reads fiction anyway, although she once loved it. Following the death from cancer of her much-loved adoptive father, Fred Norbury, Emeritus Professor of Engineering at Liverpool University, when Evie was only 20 months old, Norbury found she could no longer read fiction. "I still can't. Fiction deals with subjects that are often too painful. After the death of my father, I felt I had lost layers of skin. I couldn't withstand the depth of feeling that fiction can lead you to, which sounds rather inarticulate. Even old films, like those incredible Chinese thrillers, I have difficulty with them. Too emotional.

"For the first time in my life I started reading non-fiction, particularly travelogues and I thought, here's a genre that I could perhaps do. But The Fish Ladder came about because Evie and I had this holiday project following watercourses upstream towards their source, in England and Wales. I began a journal for her but not for publication, simply scant notes." Impressively, Norbury has now graduated from the Creative Writing MA programme at the University of East Anglia and is a doctoral candidate at Goldsmiths.

It was, though, a work of fiction, Gunn's haunting 1951 novel, The Well At the World's End, that spawned those walks and her book. The family had been living in Barcelona for several years. Finances were tight as she has not worked full-time since their daughter's birth, and the euro was weak against the pound. Thomson stayed behind to write while his wife and daughter returned to Cheshire to see her adoptive mother, Jean, before basing themselves at a family cottage on the Llyn Peninsula, in north-west Wales, for the three-month-long summer holidays.

That summer, Norbury should have been cradling her baby son, "warm and cuddly and smelling of sunshine," so she was struggling. "Life couldn't, and shouldn't stand still," she writes. Alone in their cottage, she came up with the notion that they should recreate the journey made by Gunn, following the Dunbeath Water to its source. "The idea came to me in a very roundabout way. I had lost a first edition of Robert Macfarlane's book, The Wild Places. It belonged to my husband. I had to replace it before he found out. So I wrote to Dr Macfarlane begging for a copy. He had one left and sent it with a charming note saying, 'By the way, have you read The Well At The World's End, by Neil M Gunn? I know you would love it.'

"'That's very presumptuous,' I thought. I had never heard of Neil Gunn but I read it and I loved it so much that I gave it to a girlfriend in Barcelona, thinking I'd easily replace it. But it was out of print - it's back now. I thought we could follow the journey Peter Munro makes in the book - in the event we didn't do that journey together until two years later, after my bilateral mastectomy and chemotherapy." (Mother and daughter found the well, "whose water is so clear it is almost invisible," after being given relevant map co-ordinates by Gunn's nephew Dairmid.)

Six years ago, though, she journeyed alone in Scotland. "I am dyspraxic and not good at reading maps. I thought I'd probably get lost in a Highland bog since I no longer had a copy of The Well At the World's End to refer to," she recalls. "But I came across Gunn's Highland River, and what fascinated me flicking through it was the landscape. I realised that, like Thomas Hardy, he always writes about the same features, so I could just follow the Dunbeath River to its source."

She pauses for dramatic effect. "I didn't read Highland River before I set out, although I had it with me. I didn't want to experience the river secondhand. Then I had this comical moment when I found its source - it just disappears into a hole in the ground. It literally seeps out of a hole about nine-inches wide, oozing into this primordial green soup. I couldn't believe that Gunn had written a book about such an undramatic river source and the source of himself. I sat on a tussock, turned to the back cover of Highland River where it said, "Fiction,' and I thought, 'Oh no, he's made it up!'"

Gunn writes about a loch with a white quartz beach overlooked by a blue mountain. "My Ordnance Survey map showed no mountain, no loch. I couldn't see anything but I read the last few chapters when the character, Kenn, feels he's being mocked by the banality of this hole in the ground; then he hears the water. So I set off in Kenn's footsteps, which was quite frightening, although I, too, could hear the water. Suddenly, there was a bank. I climbed it, the clouds lifted and there was the loch.

"It was like a curtain going up: a beautiful blue sky, the mountain reflected in the loch. I felt that I was an actor on a stage, part of something much bigger than me, that I was connected to, not in a romantic, conservationist way but just this sense of being alive - and that, of course, I now value more than anything."

For Norbury, it was a moment of spiritual dimension - her love of nature had been nurtured by her adoptive parents, with blissful childhood holidays roaming Scotland. She had, however, a sense when she returned from that solo walk that "something was wrong, a moment was passing." The following summer her cancer was diagnosed. After punishing treatment and losing all her blonde hair, which inexplicably has grown back a rich brunette, she is now recovered, although still taking Tamoxifen.

During treatment, she kept being asked about her family's medical history, which she did not have. "I needed it for Evie, my precious, delightful daughter. I traced my mother through an independent social worker and the curt reply came back that she didn't want anything to do with me. "I wasn't prepared for the vitriol of her response - she called me 'it'. It was beyond my comprehension, extraordinarily hurtful. I don't regret having done it because I now have answers, knowledge, information. I think who you are and who we came from is imperative to the human condition. And I do now know one of my half-brothers as I write in my book. So I have at last met someone who shares my blood. It's wonderful to have 'Robert Thomas' in our lives."

Is she happy that she discovered she had something to say, I ask, telling her that reading The Fish Ladder is like slaking your thirst at a literary well.

"Oh yes," she replies. "It really is astonishing to have this book with my name on it - I never knew my real name - and with my words and thoughts and observations out in the world."

Already working on her second book - it's about small touring circuses - she's throwing herself into the research, physically and mentally. When we part, she laughs: "Who knows? I may run away and join the circus."

The Fish Ladder: A Journey Upstream, by Katharine Norbury (Bloomsbury, £16.99).