Christopher Priest is normally categorised as a science-fiction writer. And maybe he is. And I am not one of those who will wriggle and squirm and do anything rather than admit that I read science fiction. Far from it. I do have to say, however, at the outset, that The Separation seems to me to be an altogether 'bigger' novel than might be suggested by decribing it as sci fi. Just don't let any sort of labelling put you off reading it -- that's what I'm really saying.
The second thing that needs to be said is that The Separation is sometimes described as an alternate-history novel. And that too needs more than a few comments.
First of all, a definition. An alternate-history novel (so called) is one which describes events in a world which might have come about if only someone had done something a little different.
This kind of speculation is as old as time. It was Blaise Pascal, for example, who first pointed out that, if Cleopatra's nose had been shorter (in other words, had she been a really ugly woman, with halitosis to boot) the whole course of history would have been different.
And while it is a long time since I did any serious reading about the French Revolution, I seem to remember that at least one historian commented on how differently things might have turned out if the French royal family had escaped, and had never been executed. At one point they managed to get as far as a place called Varennes, where they were recognised and forced to return to Paris. And, as I recall, one historian listed some simple but critical mistakes which the royal family might have avoided if only... Simple points which would have changed the course of history. If only...
And we cannot, of course, leave the question of 'alternate' history without pointing out that science-fiction admirers and critics do themselves no favours by using this term. In the world of recorded music, Humphrey Lyttelton long ago pointed out that the practice of referring to 'alternate' takes was a nonsense.
The correct description is surely 'alternative' takes and 'alternative' history. The adjective 'alternate' means 'every other (of two things) each following and succeeded by the other in a regular pattern' (Concise Oxford). 'Alternative' means 'available as another possibility', which is much more in line with what the record collectors and sci-fi commentators have in mind.
However, largely, I suspect, through the North American influence, we seem to be stuck with 'alternate history' as the most widely used form of words, and if you want to read a handy summary of the origins of this type of fiction you can find one here.
At last we turn to The Separation. Basically, this is a story about a pair of male twins who win bronze medals for rowing in the Berlin Olympic games of 1936. Thereafter they live in a world in which things happen which did not happen in the real world of our history, but which very well might have happened, if only...
Christopher Priest is a man who knows the true history of the second world war intimately, and is thus able to offer us variations on it in an absolutely convincing manner. Beyond that I am not going to say much about The Separation, except that it is exceptionally well written, thoughtful, and intriguing. It must have taken a very long time to write, and it is, perhaps, a little over-written in places. But it would be churlish to complain about something so subtle. (If you insist on a summary of the story, you can find one here.)
The quality of The Separation was widely recognised on its publication in 2002 (the reviews are available on the author's own web site) and in 2003 it won both the BSFA Award and the Arthur C. Clarke Award. Something strange seems to have happened in its publication history. It was first issued as a trade paperback by Scribner, but one source says that it was 'withdrawn'. In any event, it was republished by Gollancz in 2004.
All in all, I warmly recommend The Separation for your attention. One word of warning: I found the ending to be both shocking and disturbing. If you are looking for something light-hearted and full of jokes, this ain't it.