Edited by Ursula Marx, Gudrun Schwarz, Michael Schwarz and Erdmut Wizisla; translated by Esther Leslie

Verso £16.99, pp312

'These fragments I have shored against my ruin,' says a nameless voice in TS Eliot's The Waste Land. The fragments are a collage of quotations, jumbled mementos of a lost world. For Walter Benjamin, this might have been the motive of cultural history: he, too, salvaged scraps from the wreckage of culture, anthologising quotes in the hope of reconstructing a past that he knew to be irretrievable. Having fled from Germany after the Nazi putsch, he tenderly reassembled memories of his Berlin childhood in a short, episodic autobiography that is also a tour of the city during the days of the Weimar Republic. In his Parisian exile, he conjured up the vanished Paris of the 19th century.

Whereas Proust's evocation of the blissful past was as easy as eating a cake, Benjamin likened himself to 'a man digging'. Proust's enchanted reveries typically happened in a cafe or a park. Benjamin, however, was working in a graveyard and his 'spade probing in the dark loam' was likely to encounter a cadaver. Unlike Proust, he did not have the luxury of completing his mnemonic research. He had to quit Paris after the fall of France. His archive, patchily pieced together in this book, which derives from an exhibition in Berlin, was dispersed among friends and in part destroyed.

He died in the Pyrenees in 1940, probably killing himself with an overdose of morphine: he had despaired of being allowed to cross into Spain and then into neutral Portugal, from where he could have sailed to safety in America. He was only 48. The manuscripts in the briefcase Benjamin was carrying vanished. All that mattered to the authorities was his meagre bankroll, used to settle his hotel bill and the cost of his funeral. He might have been sourly or sadly amused by the fate of his treasured meditation 'On the Concept of History', which was, no doubt, binned when the room occupied by this dead transient was cleaned out.

Benjamin relished Baudelaire's description of the poet as a ragpicker, cataloguing and collating the refuse of the city and he applied the same image to his method as a cultural historian. He was a connoisseur of ephemera, like advertisements for the defunct products once sold in Paris's empty, obsolete arcades.

Eliot's poem was artificially fragmented by Ezra Pound. Benjamin did not have to pretend to be fabricating a ruin. Hustled by history and menaced by poverty, he scribbled his most brilliant perceptions on scraps of paper, some of which are reproduced in the Archive. Any tattered or expendable sheet would do: an opened envelope, train tickets, request forms from libraries, a prescription pad thrown away by a friendly doctor. Intriguingly, a note on the idea of aura - crucial to Benjamin's great essay on 'The Work of Art in the Age of Mechanical Reproduction', his elegy for the idiosyncrasy that has been expunged from our mass-produced society - is scrawled on an advertisement for San Pellegrino mineral water. In Italian, water is 'acqua'. Immediately beneath the commercial logo, Benjamin asks: 'Wass ist Aura?' A Proustian moment, perhaps: did the question fizz up in his brain as the result of a punning leap across the border between Italian and German?

To explore the submerged cultures of Berlin or Paris, Benjamin had to cram a library in his voluminous brain. He could not take his books with him when he left the Third Reich or Vichy France; the way to manage the feat was to reduce elaborate theses to abbreviated jottings. He found the same miniaturisation in the arcades. They offered customers a city scaled down, where a universe of arcane commodities - orthopaedic corsets and seed pearls, kitchen utensils and prosthetic limbs - was compartmentalised and kept in storage. Benjamin mimicked this Lilliputian economy in his efforts to save paper. His handwriting was tiny, like the refined and tremulous web of a spider. He never found a pen with a point small enough to satisfy him and often wrote with the nib upside down. He also crisscrossed pages with script, creating a palimpsest of superimposed layers. He was fascinated by labyrinths and his manuscripts often look like madly convoluted mazes.

Benjamin's ambition was to squeeze 100 lines of crabbed, compressed thinking on to a single page of notepaper. Though he never managed it, the result would have been, like The Waste Land, an epic seen through the wrong end of the telescope. His model for this imploded infinitude was two grains of wheat in the Cluny Museum, on which a believer had inscribed a Talmudic prayer. The fragmentation of Benjamin's texts was a tragic precaution. Though he preferred to use waste paper picked up along the way, he always carried a notebook. 'Let no thought pass incognito,' he counselled, adding that stray ideas should be tagged 'as strictly as the authorities keep their register of aliens'. The metaphor chillingly alludes to his own difficulties and boldly co-opts the inimical role of the border guard: the mind must trap thoughts as they slip like fugitives across the barrier between dream and waking.

The same pardonable paranoia made Benjamin experiment with encryption. When the secret police had his friend Gershom Scholem under surveillance, Benjamin proposed that they should exchange letters in code. He devised a cypher for himself and hoped to guard his anonymity by signing the works he wrote in exile as OE Tal: the pseudonym scrambles the Latin word 'lateo', meaning 'I am hidden'. The subterfuge was excellent training. Literary criticism, after all, is about deciphering texts, cracking their code in order to drag absconded and perhaps illicit meanings into view.

Given the morbidity of Benjamin's historical task and the guilt-ridden panic of his critical procedure, it is a relief to find him, in one section of the Archive, succumbing to childishness. He collected toys and delighted in the ingenious absurdity of a wooden sewing machine. He also lovingly recorded the mental growth of his son Stefan, whose garbling of polysyllabic words seemed like Joycean inspirations. Being a parent enabled Benjamin to restore his own childhood, which meant regaining paradise.

Yet the precocious Stefan was impatient for the fall. As his father's son, he could hardly wait to advance from playful innocence to the doomy, care-laden world of adult consciousness. One day, the boy asked his mother why people have heads. Exhausted by his interrogation, she was unable to reply. Benjamin might have been able to answer him. The head's purpose, surely, is to contain the past. It is our travelling archive, indexing our experience and also preserving the smudged traces of the fading world into which we were born.

It's a shame the editors of the Archive have not made access to Benjamin's cranial filing system easier: the translation is clumsy, the annotation shoddy. But there are rewards, since the plates allow us to look over Benjamin's shoulder as he scribbles nonsense verse during a mescaline trip or makes lists of the works he did not live to complete. These frayed leavings testify to the nobility of his undertaking: a lone mind battles against the death of culture and, in the process, almost forgets to fend off its own extinction.