"There was a hill that ate people," begins the first story. Just like that. "Far away, and a long time ago, on a high mountain, without trees for shelter, without body or arms for anything, on spindly legs, ran Great Head," begins another story. We are in the realm of folk tales, where we are told what happened, and we must simply go along with it.

There is something of the national treasure about Alan Garner. He has been writing excellent books for more than 50 years. He was, I suspect, the first person to write what now we would describe as urban fantasies. His prose at its best (and it is pretty much always at its best) seems inevitable, and pushes reviewers into using similes that compare it to rocks and gorges and unchanging natural formations.

As I read the Collected Folk Tales there was a feeling of happy familiarity from the first, a déjà vu, as if I knew these stories, some of them intimately. My assumption as I read was that I had encountered most of them in other forms and other places (folk tales are told and retold, after all), but then, when I finished reading, I looked at the copyright page, and realised that more than half the stories had been published in 1969 as The Hamish Hamilton Book of Goblins. I read it when I was nine, and reread it often. I could remember it in my local library, remembered taking it off to a quiet corner, remembered how much I had loved it.

It is peculiar to encounter a book half of which was assembled up to 40 years after the rest and not to be able to see any obvious difference in the writing or the writer. The prose in the old stories feels as inevitable as the new. The stories are written in a variety of voices, emulating the places the tales came from, but the prose is always spare and hard, not a word wasted, not a word out of place.

Here we have stories from Britain and Ireland and all over the world, retold with assurance. Some of the high points that were not in the original collection include "The Flying Children", a story of lies and sex and supernatural revenge and murder that I first encountered in Neil Philip's Penguin Book of English Folk Tales. It is a story that makes authors want to retell it (I shoehorned it into Sandman). Then there's a tale Garner calls "Iram Biram", which Philip called "The Pear Drum" when he collected it, and which is a curiosity in itself, because it began as a nightmarish Victorian short story by Lucy Clifford called "The New Mother". Garner strips it down to its elements. It's an act of literary ventriloquism that illuminates the oral and folk tradition. Two girls named Blue Eyes and Turkey are tempted by a wild girl to be naughty, with the promise of a gift of a mysterious "pear drum". They are not naughty enough to get the drum, but are still so naughty that their mother leaves, and a new mother, with glass eyes and a wooden tail, takes her place.

Garner goes beyond the original ending, playing with the sound and the meaning of words:

There were no lamps lit, but in the glow of the fire they saw through the window the glitter glitter green glass of a mother's eye. They heard the thump; thump; thump of a wooden tail.

Iram, biram, brendon bo
Where did all the children go?
They went to the east, they went to the west
They went where the cuckoo has its nest.
Iram. Biram. Brendon. Bo.
And the Wild Girl wept.

Garner makes up a poem, and adds the haunting image of the Wild Girl weeping as a way of closing the tale, thus moving it somewhere entirely new, away from Victorian nursery horror and into the realm of the twice-told tale.

The book itself has a core of goblin stories  but a goblin can be anything, and Garner's own preferences seems to be for moments of the inexplicable. So many of the tales lack explanation for the events in them, as if the stories were the lyrics of folk songs, and the true meaning is in the music. There is an essay in here on the roots of the fairy folk, and who they really were, or might have been. There are a few dialect pieces, written, Garner says, in the voice of his blacksmith grandfather. There are poems: some collected from "Anon" and the dead, others by Garner himself, my favourite being "R.I.P", which begins: "A girl in our village makes love in the churchyard. / She doesn't mind who, but it must be the churchyard," and continues, lusty and honest to its bitter-hopeful end.

There is a version of Valmiki's Ramayana. There are Norse gods, Algonquin revenge magic and a hero's odyssey from Ireland to a series of magical islands.

This Collected Folk Tales is, by definition and by temperament, a patchwork, and reading it is like entering a rag and bone shop in which every object has been polished up and repaired and made fit for use, while always leaving in the cracks and dents that show that the goods have had years of use already. With the exception of some of the poems, there is nothing new or shining here, and the book is all the better for it. If I had small children, or a class, I would read to them from it.

And if, by the time I have grandchildren, there are still public libraries, as I hope there will be, I trust that they will find this book themselves in one (for it will be all the better for not being given or suggested or recommended to them by an adult), and take it to a quiet corner and read.

 Neil Gaiman's Odd and the Frost Giants is published by Bloomsbury.