Van Morrison’s music as I hear it holds a story – a story made of fragments. There is in his music, from the very first, a kind of quest: for the moment when the magic word, riff, note, or chord is found and everything is transformed. At any time a listener might think that he or she has felt it, even glimpsed it; a realm beyond ordinary expression, reaching out as if to close your hand around such a moment, to grab for its air, then opening your fist to find a butterfly in it – but Morrison’s sense of what that magic moment is must be more contingent. For him this quest is about the deepening of a style, the continuing task of constructing musical situations in which his voice can rise to its own form.

"When I was very young," the late Ralph J Gleason wrote in 1970 in a review of Morrison's album Moondance, "I saw a film version of the life of John McCormack, the Irish tenor, playing himself. In it he explained to his accompanist that the element necessary to mark the important voice off from the other good ones was very specific. 'You have to have,' he said, 'the yarragh in your voice'." To get the yarragh, for Morrison, you may need a sense of the song as a thing in itself, with its own brain, heart, lungs, tongue, and ears. Its own desires, fears, will, and even ideas: "The question might really be," as he once said, "is the song singing you?"

His music can be heard as an attempt to surrender to the yarragh, or to make it surrender to him; to find the music it wants; to bury it; to dig it out of the ground. The yarragh is his version of the art that has touched him: of blues and jazz, for that matter of Yeats and Lead Belly, the voice that strikes a note so exalted you can’t believe a mere human being is responsible for it, a note so unfinished and unsatisfied you can understand why the eternal seems to be riding on its back.

Morrison will take hold of the yarragh, or get close to it, raise its spectre even as he falls back before it, for the moment defeated, with horns, volume, quiet, melody and rhythm and the abandonment of both, in the twist of a phrase or the dissolution of words into syllables and syllables into preverbal grunts and moans. He will pursue it perhaps most of all in repetition, railing or sailing the same sound 10, 20, 30 times until it has taken his song where he wants it to go or failed to crack the wall around it.

The yarragh is not, it seems, something Morrison can get at will, or that in any given year, or even decade, he is even looking for; the endless stream of dull and tired albums through the 1980s and 1990s, carrying titles like warning labels – Beautiful Vision; Poetic Champions Compose; Avalon Sunset; No Guru, No Method, No Teacher; Inarticulate Speech of the Heart; A Sense of Wonder; Enlightenment – attest to that.

So do a string of records where Morrison seems to attempt to reduce whatever might be elusive, undefinable, and sui generis in his music to parts that can never recombine into a whole, as he recorded jazz and jump blues with his sometime accompanist Georgie Fame, country with Linda Gail Lewis, Jerry Lee's sister, traditional Irish songs with the Chieftains, and, most touchingly, even skiffle with Lonnie Donegan himself, back in Belfast: two old men standing up to the crowd to sing Midnight Special, not so far from where, once, one of them named a band after it.

Those are episodes in a career. It’s the fragments of music as broken and then remade by the yarragh that this book is looking for.

“The only time I actually work with words,” Morrison said in 1978, “is when I’m writing a song. After it’s written, I release the words; and every time I’m singing, I’m singing syllables. I’m singing signs and phrases.”

The quest for the yarragh – for moments of disruption, when effects can seem to have no cause, when the sense of an unrepeatable event is present, when what is taking place in a song seems to go beyond the limits of respectable speech – is also a performer’s quest to evade and escape the expectations of his audience.

It’s a struggle to avoid being made irrelevant and redundant, a creature tied, as if by chains, to his hits of 40, 30, 20 years earlier – forbidden, by the laws of the pop mind and the pop market, from ever saying anything he hasn’t said before.

From the time of his first hits Morrison has, in a way, set himself against any possible audience: he does his work in public, but with his back turned, sometimes literally so, and it might go back to those nights in the Maritime Hotel in 1964.

“Out of nowhere, these kids began showing up,” Morrison said in 1970, and I remember the way his eyes sparkled in a set, stolid face as he talked: “Sometimes, when it all worked, something would happen, and the audience and musicians would be as one.”

That was because no one knew what might happen and no one knew what was supposed to happen. Before a song is recorded, there is no right way; afterward, especially in the pop glare, audiences know what to expect and expect what they know.

Van Morrison, then, is a self-contradictory individual whose work is about freedom. How do you get it? What do you do with it? How do you find it when it disappears and what is it? Is the yarragh the means to freedom, or is it, when you can find it, the thing itself? When Morrison reaches the moments of upheaval, reversal, revelation, and mirror-breaking – in his music and, sometimes, in what other people have done with his music, finding, as Neil Jordan would do in his film Breakfast on Pluto, a yarragh behind the yarragh, or dramatising it as Morrison might not –all of those questions are thrown into relief. It becomes plain that any summing up of Morrison's work would be a fraud.

That is what makes his failures interesting and his successes incomplete; it’s what allows the most valuable instances of his music to exist less in relation to other instances in his career in any historical sense than in a kind of continual present.

This is an edited extract from Greil Marcus’s book When That Rough God Goes Riding: Listening to Van Morrison. Reproduced by kind permission of the author