I had no idea people were still writing sf quite like All Of An Instant. This could have been written 60 years ago or yesterday, and is reminiscent of a bizarre cross between Stapledon's Star Maker and of Edwin Abbott's Flatland. The former for it's immense scope, the latter for the single weird idea taken to it's limits.
The single weird idea is that time is split into two layers, likened to earth and water. You and I live in the earth layer (or Flux), prisoners of linear cause and effect. "Above" us lies the water layer (or Instant) where every moment of time in the earth layer exists simultaneously. The Instant has been progressively colonised by tribes of humans who have learned how to use its ripples and currents to alter history back in the Flux, with the result that the Instant is now in complete anarchy as hundreds of tribes fight to stabilise their preferred version of history in place of others'.
We are rather leisurely introduced to the three primary characters (or, more correctly, I suspect, archetypes) in the narrative: Nir, a War Chief of the Ghosts, sworn protectors of the single mother of all humanity many thousands of years ago, Kookatchi, the legendary greatest thief of the Instant and Quillithé, the Lady of the only army of the Instant. The appearance of an inexplicable phenomenon in the Instant portends the onset of a catastrophe against which they must combine their knowledge and transcend their prejudices. That's the plot; any more detailed synopsis would only end up swallowing the rest of this review.
All Of An Instant is a simple idea, complexly realised in a beautiful semi-mythical style about which I freely admit to being rather dubious after reading only the first page, since Garfinkle's prose seems deceptively simplistic. But it is simple rather than simplistic, flowing easily, letting you forget just how bizarre the world you're reading about is until suddenly you have to reread a whole page s-l-o-w-l-y. There's no difficult words, no experimental style or stream of consciousness, it's the concepts that can confound the unwary (i.e. me).
The writing style might grate a little at first, but you soon realise that there's simply no other way of writing a book like this. It isn't hard sf, isn't traditional fantasy, in fact doesn't really read as anything except, perhaps, myth. My one complaint about All Of An Instant might be that it is over