August Brill, a widowed book critic in his seventies, is laid up at his daughter's house in Vermont after breaking his leg in a car accident. Also staying there is his granddaughter Katya, traumatised by the horrible death of her former boyfriend in Iraq - or rather by the video of it that the kidnappers posted online.

We join August for one sleepless night. While he tosses and turns, or drinks the odd glass of water, he sometimes thinks back on his marriage, but mainly he makes up a story to pass the small hours. In it, a man called Brick wakes to find himself in an alternative America racked by a second civil war. Brick is told he has to end the conflict by assassinating none other than August Brill, who started the whole thing in his imagination. If Brick refuses, then the secret agents who have given him the mission will kill him and his wife.

Apart from some quasi-Stephen Hawking notions about alternative universes (“There's no single world. There are many worlds and they all run parallel to one another, worlds and anti-worlds”), it is far from clear at first sight what Paul Auster means by all this. For one thing, the tough secret agents possess guns, August's address, the ability to travel between dimensions and the uncritical willingness to kill in cold blood. So, and you may be ahead of me here, why don't they just go and kill August themselves? Why do they make their job more difficult by trying to co-opt a reluctant amateur? Is this the incoherence of night thoughts, or a clue as to why August is only a critic and not a novelist, ie he can't plot for toffee?

It hardly matters, since even August gets bored and gives the story an arbitrary ending halfway through. As a further distraction he recites the plots of several classic films he's watched with Katya, to point up a theory she has about film narrative. These bits are highly readable and engaging. Auster's cool, economical, rather European prose style, which suggests he has spent a lot of time reading Italo Calvino, Peter Handke and such, quite makes up for the derivative content.

Eventually Katya, who can't sleep either, comes in and August tells her all about how he met grandma - in detailed terms. “The body has a limited number of orifices. Let's just say we explored every one of them.” This seems fairly unlike the way a grandparent and grandchild would generally care to converse. But I suppose you never know, with families.

When Katya nods off, August recalls the ghastly beheading video, which they watched out of duty to the murdered aid-worker boyfriend, “so as not to abandon him to the pitiless dark that swallowed him up”. But presumably we are all, like August, in the dark, since this life is the Valley of the Shadow of Death. Man in the Dark, seems, in fact, to be a meditation on death, and perhaps the transformative power of art and love in the face of death.

Despite its bittiness, the novel becomes oddly shapely and satisfactory by the end, always helped by the unity of time and that cool style. And points should, of course, be awarded for making a book reviewer the hero. August is a pretty exalted critic, making a good living, with a Pulitzer prize to his credit. But that is no more unlikely than the bestselling success that attends Auster's own slim, thoughtful volumes.

Man in the Dark by Paul Auster
Faber £14.99 pp181

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