Will Self's sprawling new novel is not his most accessible but it may well be his best

In 1971, Audrey Death has spent half a century catatonic in Friern mental hospital. Zack Busner – a radical psychiatrist just recovering from coming under the influence of RD Laing – has an idea. He recognises that she and several of the hospital's other long-term patients may not be mad, but are actually suffering from the 1920s sleeping sickness encephalitis lethargica. Oliver Sacks-style, and somewhat outside the formal channels, he tries treating them with L-dopa. They start waking up.

Meanwhile, if that's the word, we go back to Audrey's wartime past as a munitions worker in the Woolwich arsenal, and those of her older brother, Albert, a war office civil servant with an eidetic memory (like Zack Busner, another old Self theme), and her younger brother, Stanley, who goes off to fight in the war. A third strand of the story shows us Busner, in old age, shambling around north London and looking back on his life – a journey that is to culminate in a visit to the now-decommissioned mental hospital.

Umbrella is not, be warned, altogether easy going: 400 pages of unbroken stream-of-consciousness dotted across three time frames, leaping jaggedly between four points of view, and with barely a paragraph break, let alone a chapter heading. I started out wondering whether, as a prank, Self had decided to write the novel that Richard Tull, the unpopular writer in Martin Amis's The Information, is working on. Self's sentences themselves sometimes resemble Victorian lunatic asylums refitted for 1970s use: ugly, overstuffed, clattering with the moans of lunatics and bristling with redundant gothic spires. The antic gurgles of laughter you find in Self's earlier work are few and far between. In their place, though, is a sustained depth and seriousness, and an ambition of technique that I haven't seen in him before.

I don't mean to put you off. Umbrella is old-school modernism. It isn't supposed to be a breeze. But it is, to use the literary critical term of art, kind of amazing. To frame it in terms of the film Gremlins, if you fed Alan Hollinghurst's The Stranger's Child after midnight, it might come out looking a bit like Umbrella. Both share the historical range, the multiplication of perspectives, the interest in architecture and memory, and the central preoccupation with time. But where Hollinghurst's mode is fastidiously naturalistic, Self's is the exact opposite. Hollinghurst is interested in time's arrow; Self presents time as a dangerously faulty artillery shell.

It's hard to précis what happens in Umbrella, because it doesn't quite work like that. It's arranged like a vorticist painting: a fractal proliferation of rhymes and symmetries; a frozen explosion. Hospital corridors are overlaid on London streets, are overlaid on trenches at the front. The packing of an artillery shell rhymes with the encapsulation of an antipsychotic drug or the manufacture of an umbrella.

The encephalitis patients are frozen in time – but in their myriad tics and autonomic circulations of the hospital they encode their pasts. Audrey's tics see her turning artillery shells on an invisible lathe or typing dockets at an umbrella factory. Busner sees what they are doing only when he films them and slows it right down or speeds it up, capturing "a flirtatious gesture that it took her two hours and twenty minutes to make". Ticcing "links macro-and micro-quanta".

Song fragments recur, tic-like, in Busner's head as in Audrey's (a nod to Pynchon, perhaps), and – perhaps too showily; Self does like to tell you what he's up to – shards of King Lear, Hamlet and TS Eliot spike the text. Memory and hallucination interpenetrate, and the narrative cuts between time streams mid-sentence. Busner copulates with a medical technician in a mechanically detached way (Ballard's a presence here) – her sexual responses analysed in clinical terms that then give way to the vocabulary of trench warfare. This is a world of constant recursion, of repetition compulsion, of the paranoid-schizophrenic apprehension that everything is connected.

Crucially, the book also contains human beings – Audrey, Busner, and a supporting cast of characters on whose bones real flesh is to be found – and is anchored in something approaching reality by a stupendous density of research and observation. Self's London or, rather, Londons are horribly alive – period details crammed in like a cabinet of curiosities. The story of Audrey's younger brother, Stanley – a machine-gunner in the war – contains some extraordinarily well-imagined battlefield descriptions:

"Pull, rotate, pull, rotate … so the belt-filler makes use of its animal components: pull, rotate … pull, rotate … On the far side of the tumbled-down fence the jellyfish of camouflage netting rises and falls soundlessly … in this ocean of noise – the gunners, stripped to the waist, scamper about the heaving creature, their devil's tails of braces bouncing on their backsides."

Read it. Then go and have a bit of a lie-down. It may not be his easiest, but I think this may be Will Self's best book.