How close I came to overlooking this book. From a heap of neglected and not-so-neglected works resurrected by Canongate, I chose this because it looked like the runt of the litter. And what kind of a serious writer calls herself "Nan"? But there was an anonymous quote from this very newspaper on the back: "The finest book ever written on nature and landscape in Britain." A large claim, I thought. We'll see about that.

After about three pages, I began to see why those words had been written. After about 11 pages, by the time she had described seeing a view from high up a mountain, I was giddy with something halfway between delight and vertigo, and wondering whether calling it "the finest book ever written on" etc was not in itself somehow mealy mouthed and inadequate praise.

I will quote the bit that made me realise what I was strongly suspecting  that I was reading a work of the highest calibre: "In no other way have I seen of my own unaided sight that the world is round. As I watch, it arches its back, and each layer of the landscape bristles  though 'bristles' is a word of too much commotion for it. Details are no longer part of a grouping in a picture of which I am the focal point, the focal point is everywhere. Nothing has reference to me, the looker. This is how the Earth must see itself."

If you have a beating heart, you will have inadvertently paused to take in the simple majesty of that last sentence. Page after page, line after line, contains something as arresting as that, as plain and bracing as the water of a mountain stream. (About which she writes amazingly, too.) When she is not doing that, she is making magic before your eyes with the English language. One reads something extraordinary, and then, in language only slightly less extraordinary, she shows you how she did it: "In the darkness one may touch fires from the Earth itself." Eh? Then: "Sparks fly round one's feet as the nails strike rock, and sometimes, if one disturbs black ooze in passing, there leap in it minute pricks of phosphorescent light."

From which detail you may deduce that this was written, or experienced, at a time when hob-nailed climbing boots were the norm. (I am tempted to buy or make a pair of my own to experience this phenomenon for myself.) The manuscript was completed in 1944, she showed it to a friend, who although loving it wondered whether it might not need a map and some photographs. And added that it might be difficult to find a publisher.

Shepherd sent it off once, received a polite letter of rejection, and then left it in a drawer until 1977, when Aberdeen University Press printed a small edition. And there it might have been forgotten, but Robert Macfarlane was introduced to it by "a former friend" (as he rather darkly puts it). "I read it, and was changed," he says in his first-rate introduction (I can think of no higher praise than to say it stands up to Shepherd's prose).

If you read it, you too will feel changed. This is sublime, in the 18th-century sense, when landscapes like these were terrifying. And she achieves it in language that is almost incantatory, like a spell: "... birdsfoot trefoil, tormentil, blaeberry, the tiny genista, alpine lady's mantle ..." runs one short list of the local flora, and it was only on rereading that I realised I had never heard of one of these flowers before, or could tell what they looked like.

But while reading it, they are  because of the power of the surrounding writing  fleetingly visible. Does that sound like nonsense? I hope not. (And I will fight anyone who says this book is nonsense. It's common sense, cubed.)

To intrude a note of coarseness, £10 might seem a lot for a book barely over 100 pages long. But it's not: it's over 1,000 pages long, because you are going to read it at least 10 times  and find something different in it each time. It's like Shepherd's landscape, constantly renewed: "However often I walk on them, these hills hold astonishment for me. There is no getting accustomed to them."