W G Sebald is England's - and East Anglia's - great German writer. As he turns his sad eye on Suffolk, Carole Angier hails a visitor from a melancholy planet
We are getting W G Sebald in a strange order: first his third book, now his fourth, The Rings of Saturn (translated by Michael Hulse; Harvill, pounds 15.99), next year his second. And yet it is probably the right order. For The Emigrants, with which we began, is the key to Sebald: the source and real subject of all he writes.

Sebald is a German of the "second generation", born in 1944, who has lived in England since 1966. The Emigrants, published in German in 1992, arrived in England four years later. If you have not read it, do: for it is a masterpiece. It consists of four portraits of exiles from Germany, not all Jewish, but all driven and destroyed by the central event of modern German history, the genocide of the Jews. It is about many other things - memory, art, literature, loss. But it makes the Holocaust as permanently present to you as it is to Sebald.

The Emigrants provides a relatively easy introduction to reading Sebald. His long, spiralling lines grow under your eyes like vines. They are dense, allusive, but of an astounding beauty: in Suffolk's past "the white flecks of the windmills lit up the landscape just as a tiny highlight brings life to a painted eye". They are above all sceptical, constantly reflecting the inability of any single sentence to be true. This is also the effect of the extraordinary structure of his books, which is like wrought iron, at once solid and fanciful: a detailed, descriptive text, studded with photographs, which tells you that it is history, or reportage, in any case non-fiction; with an intense unity of tone and vision, which tells you instead that this is a brilliant, private, fictional creation.

The Emigrants was both: the histories of real people; but transformed by details that flow from elsewhere, especially his reading. Sebald's primary schoolteacher is laced with bits of Wittgenstein; the artist Max Ferber draws his history from a friend of Sebald's, but his artistic life and style from a famous painter about whom Sebald had read. Not all the photographs are what they purport to be: the one of "Max Ferber", for instance, is - so Sebald tells me - of neither of his models.

The Rings of Saturn is the same: reportage and history pinned to reality by photographs; yet an imagined work, even a dreamed one: not a view but a vision. As in The Emigrants, two things inform this vision: the glories of literature and the horrors of history.

Sebald walks in a loop around Suffolk, down the coast from Norwich to Orfordness, and inland back to Norwich. The loop of his walk is itself a Ring of Saturn: Saturn being the Planet of Melancholy, its rings made of ice - fragments of a previous destruction.

Sebald describes for us recent wars, enslavements and genocides up to the First World War, all subterraneanly connected to the peaceful Suffolk landscape. Then, like flecks of light in the darkness, he evokes saving human presences, or more often absences: friends; great writers who lived or died in the places he visits, and whose lonely lives and lovely words he relates: Thomas Browne, Conrad, Swinburne, Edward FitzGerald, Chateaubriand.

These small biographies are beautiful and moving. I especially loved Swinburne, with his huge head and fiery hair, and FitzGerald, who goes to live on water, "where no friends are buried nor Pathways stopt up"; the Irish girl Catherine Ashbury, who sews a bridal dress she will never wear; and the Suffolk gardener William Hazel, who cannot forget the wartime bombers taking off to set fire to Germany, until the whole country "was burnt into me". But they are, I warn you, even sadder. There were only four sad stories in The Emigrants: you could hope the shadow would not fall on everyone. In The Rings of Saturn it has fallen on everyone. We meet 40 people, almost all marooned on the Planet of Melancholy, gripped by its icy rings.

The emigrants had lost their whole lives, but they had not lost the love of those lives. That was why they were so sad; and why The Rings of Saturn is sadder. On the Planet of Melancholy life is not loved. Sebald writes glorious elegies for the lost works of man: medieval Dunwich at the bottom of the sea; the great houses of East Anglia crumbling to dust. But soon we read that Somerleyton Hall is only lovely now, on the brink of oblivion; that all those vivid places were only the wages of power; that Thomas Browne, Edward FitzGerald, poor Algernon withdrew from the world then, as Sebald withdraws today. It is not that you do not assent, under the spell of this darkly shining prose. You do; and the shadow falls on you.

This sounds like a warning not to read The Rings of Saturn, which is the opposite of my intention. I hope everyone will read it - it is very beautiful, and it takes you inside a world poisoned by evil. It is also arcane and erudite. Sebald's prose, like Eliot's poetry, is such a tissue of quotations that he was once asked if he simply transcribes them. Of course he distils and transmutes them; but they still stand inside the lines like a sculptor's armatures. In The Rings of Saturn, he says that he wanted to identify the sources of his thoughts. And so he does: showing us his own voice crossing with Swinburne's, FitzGerald's; and with Kafka's, Borges's, Stendhal's. But he cannot identify everyone; as he unravels part of the magic carpet behind him, he weaves the part ahead. Some voices we can recognise - the English ones, such as Shakespeare's. Others I could never have guessed. This makes his work more echoing, more mysterious. And it is the objective correlative of his theme: his (and all Germans') loss of the past. These fragments he has shored against their ruins.

Since The Rings of Saturn is really about Sebald's own mind, it must contain two other subjects as well: art and genocide. It has many beautiful and melancholy things to say about art, and about the dreams and costs of making it, several of which revolve around the central image of silk: Catherine Ashbury's wedding dress, for instance, which is made of hundreds of scraps of silk; or the 18th-century silk pattern books which Sebald finds in Strangers' Hall in Norwich. Most beautiful of all are the moments of Proustian epiphany, in which the past is retrieved in imagination: when a visitor to Swinburne in old age, listening to him tell a tale heard in childhood, clearly sees both the fiery-haired listening boy, and the dark figures of the story; or when Sebald himself, dazzled by the setting sun, suddenly sees the sails of the long-vanished windmills turning.

And that genocide? Of course it is here. There are echoes of it in the story of silk; and in Sebald's trip to Orfordness. At one point, however, it breaks the surface: in the story of Major Le Strange, who grows more and more odd and reclusive. When he dies, he leaves his land to his housekeeper, with whom he had taken his meals in silence for 40 years. A clipping on the story appears opposite Sebald's version; it includes the detail that Le Strange had taken part in the liberation of Bergen-Belsen in 1945. But this clipping, Sebald tells me, is also one of his "falsifications". He had read a similar story in the local paper; but the man had not been destroyed by the memory of Belsen. Sebald added that meaning. He always does.