Alan Garner is a few days from his 90th birthday when we meet, and his plan for the day itself is “to be very quiet”. He says, “I sound antisocial but I’m not. I’m very sensitive to people and I don’t like more than three or four people in a room at a time.” Since The Weirdstone of Brisingamen, published in 1960, he’s had a long and singular writing life, with a certain amount of gregariousness forced on him by its extraordinary late flowering over the last dozen years.

There was the surprise of a conclusion, half a century on, to those very first children’s fantasy novels: in 2012’s Boneland, the adult Colin seeks answers to childhood mysteries through astronomy, therapy and quantum physics. There was a vivid child’s-eye memoir, Where Shall We Run To?, setting down Garner’s wartime primary school years in rural Cheshire. Three years later he wrote the Booker-shortlisted Treacle Walker, which, in drawing on his childhood as well as local landscape and legend, seemed to distil the whole arc of his literary career into one riddling, playful, dizzyingly deep novella. And now comes Powsels and Thrums, a collection of essays, poetry and short fiction that ranges across his life and work, and showcases “a side of me I’ve used in research that has never appeared until now”.

“Powsels and thrums” were the multicoloured snippets of cloth left over from handloom weaving, which Garner’s craftworker ancestors “brought together to make something whole and new”. Similarly, the collection brings together treasured memories of his grandfather, Joseph Garner, a gifted triple smith (white, black and lock), and of secondary school, where the head, Eric James, became the second great influence on his life. It includes an account of Garner’s chance discovery and painstaking renovation of his medieval house, Toad Hall, found as rundown cottages in 1957 after he spotted a “for sale” sign in a hedge; thoughts on creativity, language and translation; and investigations of the legends that have fuelled his fiction, and the remarkable archaeological discoveries to which those legends have led.

Archaeologists come to me and say: ‘What do you think,’ and I see the answer – that’s the novelist working

It has been a life marked by great serendipity, and painful rupture. A clever, sickly child who survived several brushes with death, at 11 Garner won a place at Manchester Grammar School, which opened new vistas while alienating him from the family who had lived and worked on one patch of land around Alderley Edge for hundreds of years. Though it came at a severe cost – “you tended to become a pariah to your own and an upstart to the community” – in Powsels and Thrums he celebrates “the Golden Age of the 1944 Education Act”, which transformed the fortunes of working-class children by opening up secondary and tertiary education.

Garner found himself on a “conveyor belt” to studying classics at Oxford, and assumed he would become a professor. Once there, “I was having a terrific time – but there was something wrong.” He wanted to make something that only he could create; as his grandfather always said, “If the other feller can do it, let him.” He left without a degree, returned to Cheshire and dedicated himself to writing. He began with the material of his own childhood, but his books swiftly resisted categorisation: spiky reinventions of myth in The Owl Service and Elidor; a spare, millennia-straddling drama of first love and mental disturbance in Red Shift; stories about his ancestors’ lives in The Stone Book Quartet.

We are talking in the vast chimney of The Old Medicine House, an exceptional Tudor building Garner saved from demolition in 1970 and brought timber by timber from a site 18 miles away, linked to Toad Hall by a modern extension. Big enough for Garner’s well-worn leather chair and an open fire basket, the chimney will be familiar to readers of Treacle Walker as the charged liminal space – both cosy and otherworldly – where the young Joe Coppock sits and chats with the titular rag and bone man. Joe has “the nuisance quality of me”, says Garner. “The child that’s always asking why.” At the bottom of the garden, trains whiz by: Garner hears the silence if they stop running.

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We are in a good place, he tells me: “a site that’s been occupied since the end of the last ice age, and just two fields away there is one of the world’s great marvels, the Jodrell Bank radio telescope. I have close friends there, and we speak the same language. The dichotomy between art and science is false, and Jodrell Bank is actively working to heal that perceived rift.”

Garner and his wife, Griselda, remain touchingly enthusiastic guides to a place they’ve lived in for so many decades, directing a torch on protective marks carved into wooden beams, messages from centuries past. The various archaeological finds the house and fields have offered up are displayed throughout, along with the artefacts behind Garner’s novels: the patterned plates from The Owl Service that can generate either flowers or owls; the hand axe in Boneland that links Colin to the stone age shaman who is his distant brother; the medicine pot from Treacle Walker. The buildings are now too valuable to keep in the family; they have been made part of an educational trust, with Garner “the last tied cottager”.

At 90, Garner is happy to admit that he’s mellowed. “I’ve decided, rightly or wrongly, that having lived as long as I have, there’s no point in taking people by the scruff of the neck any more. It will sort itself” – which links, he says, to why he’s apolitical: studying ancient Greece and Rome gave him too long a view on current affairs. The peaks and troughs of the bipolar condition he lives with are also “levelling out, with age”.

In hunter-gatherer societies everyone is equally valuable, which we have lost totally

“I have no problems with being a few days away from 90,” he continues, citing what he learned from researching Strandloper, his 1996 novel in which a Cheshire labourer, deported to Australia at the begin­ning of the 19th century, joins an Aboriginal community. “In hunter-gatherer societies, every member, perforce, is perfectly suited for doing something for the benefit of the group. You can put a three-year-old child into a hole to see if there’s a wombat, and the old men, whose knees are gone, sit under the trees in the shade and look at the sky and they tell the young men where the game is likely to be. Everybody is equally valuable. Which we have lost totally.”

Garner was delighted to find himself on the Booker shortlist for Treacle Walker – “both for myself and all the people who support me. But I wished it could just be a shortlist. To say anything is ‘the best book’, to me, that’s intellectually impossible. All reading of fiction is subjective. It’s a false premise that art is competitive; it ain’t.”

It could have been his final book. “I finished Treacle Walker and I was content. Then I looked back and I thought: no, I’ve got to put the two halves [of my life] together. I’ve got to do right by my grandfather.” Powsels and Thrums is a bringing together of two cultures: the oral culture his grandfather imbued in him, with a deep knowledge of locality and craft, and the world of academia. “I didn’t see until I was right at the end of writing it, that two sides of me were distilled into two human beings: Joseph Garner, and Eric James of Manchester Grammar School. Who as human beings focused me, but also it was what had made them, and I was the rich beneficiary of both those great cultures. It was up to me to integrate them. Powsels and Thrums is an acknowledgment of my great good fortune in being alive at that moment to benefit freely, in every sense of the word, from those two sides of my background. Which is not there now.”

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The key inheritance from his grandfather has been the Legend of Alderley, told to the child Garner in a matter-of-fact way, in Cheshire dialect, as another local anecdote. “I never said to him: ‘Grandad, is this true?’ I think if I had, he would have sworn to himself and thought, ‘I’ve told the wrong person.’” The legend describes a farmer with a horse to sell, led around various local landmarks by a mysterious stranger, who eventually discovers a sleeping hero under the hill (versions of the story appear across the world, and are strongly associated with Arthurian myth). It informs many of Garner’s novels, from The Weirdstone of Brisingamen to Treacle Walker. In Powsels and Thrums, he describes how as an adult he examined the legend with an archaeologist’s eye, pursuing fragments of memory from the early bronze age.

“My grandfather and I had the landscape in common, and could both look at it from our different angles. When the academic in me was there, I could see the story was full of anomalies. I was able to pursue the anomalies, leading to archaeological discoveries, as a result of which Alderley Edge was recognised as the second-oldest metalworking site in England.”

As Garner was putting the book together, “I realised that there are two things in my life that really, really matter to me. One is, I had actually, finally, managed to write Treacle Walker – though I didn’t see that coming. But after Treacle Walker, what? I don’t know. It’s the only book that I haven’t yet got round to criticising savagely.” The second is the confluence of his grandfather’s stories and his own research. “Because of my academic nature and background, I was able to take [on] something that had been given to me as an oral tradition by my grandfather. It has filled me with awe, because if it hadn’t happened then, it would not have happened again. I was of the right generation, in the right place at the right time, to be able to put it together. The chance is amazing.”

Famously, Garner has always “wallowed” in research. “All my novels are the result of academic method, that’s why they were so long in the writing. You’ve got to choose the right ingredients, put them into the still, and let them work. And then they deliver themselves.” It’s “not a mystical process, but it can feel quite mysterious”. He describes archaeology not so much as his parallel career, but his “parallel universe”. “I’m not a qualified archaeologist but I have archaeological abilities. Archaeologists come to me and say: ‘We’ve got this and we’ve got that, what do you think,’ and I see the answer, which is totally irrational. That’s the novelist working.”

Another essay in Powsels and Thrums charts the archaeological groundwork for his 2003 novel Thursbitch, set around a valley in the Pennines, a place of such dark energy that local farmers stayed indoors at night and the vicar of the parish wouldn’t visit it. His own unnerving experiences there included photographing a large block of stone, and coming back later to find it impossibly altered. “A particle physicist friend just said: ‘Oh, it’s a time slip.’ He could slot it into quantum theory. ‘Very interesting, Alan, but quite normal.’”

The book is punctuated by his grandfather’s dry sayings – “Tha pisses more than tha drinks” – and also by Garner’s poetry, another parallel universe to his prose. “It’s almost like automatic writing. I don’t like prose being analysed and I like poetry being analysed even less. There are a couple of things in there that I just don’t understand, but I know they’re right. They just came. Inevitable.” He also shares his theory on the genesis of Lewis Carroll’s nonsense poem Jabberwocky, in which he sees memories of the Cheshire dialect Carroll would have heard from servants as a child. “It was a lightbulb moment. He was brought up with these sounds around him and probably never saw them written down or even understood their meaning.”

One poignant fragment recalls going running with Alan Turing, when the teenage Garner was a schoolboy athlete, before the police ordered Garner not to associate with him – “something I don’t think I shall ever fully recover from. We shared a very quick sense of humour, a love of punning and a delight in being mischievous and quite scatological. He was very adolescent in that way.”

Throughout the book, Garner also explores the visionary imagination, delving into the roots of creativity in the unconscious mind. The process is necessarily veiled, meaning that the question of whether he’s working on something new can only be answered with: “Not that I know of.” His novels have always channelled ideas about time and quantum reality, and he is keen to elide distinctions between art and science: working with Jodrell Bank on what he calls “Operation Melting Snow”, and today describing maths as philosophy, philosophy as a game, creativity as play. What Garner knows for sure is that “I don’t write set books. I keep coming back to the distinction between mysterious, which is OK, and mystical, which is not OK. The thing that ties all creativity together is not something that universities should analyse, but people should just accept as wonder.”

Powsels and Thrums by Alan Garner is published by 4th Estate. To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.