The reason why it was so valuable not knowing about the content of the book is not because of any particular twist but, on the contrary, because of the lack of a twist.  The information is provided to the reader so carefully that the potence of the information (how it might have been a twist) only becomes apparent later.  Ishiguro's narrative is not just controlled: it is almost fanatically managed.  

Kathy H. and her friends are clones, who have been brought up to provide organ donations; usually, after four donations, the clones "complete".  It is never stated what happens after "completion", but there is a terrible hint that the word is at least partly inaccurate.  Much of the book follows them as children at Hailsham, a special school for clones where Kathy becomes friends with the domineering, prickly Ruth, and eager, easily goaded Tommy.  They know what they are bred for, and they know what they are to do – this is not a novel about escape and freedom (indeed, it is painfully poignant that the characters' biggest dream is of a three year deferral).

The process by which these children learn about their fate is elegantly described: they are given just a little bit more information than they can developmentally handle at any point, and so when things become clear, later on, it is as if they always already knew it, and there is no shock, no surprise, but instead a sense that this is natural.  Ishiguro does the same thing in how he writes the novel.

The reader is frequently informed that Kathy, as she describes her memories, will return later to explain some event she just mentioned.  And when she does, the moment is never as shocking as it might have been, because the twist itself is already known, already passed by in space and time.  Ishiguro does not just do this with plot points: he does it with character development and with aspects of the clones' lives, such as their sexuality.

The effect is such that this extraordinary novel comes across as a type of realism, and that shocking revelations are not shocking – even more importantly, the vast unknowns that are never described seem to disappear, as if they are held up to the reader's blind spot (for example, what does "complete" mean?  There is the hint that there is survival beyond that fourth and final donation  What parts are donated?  What exactly is donated?)  

As a social satire, there is also a richly observed commentary: about the idealism and energy of the 1970s, and how these were chipped away at and lost by the pecuniary conservativism and the moral insularism of the West in the 1980s and 1990s.  But it is at its most gut-wrenching as a type of empty autobiography: the novel is like a nail dropped into a bucket: a loud, empty, clanging sound.

This book may be fascinating for medical ethicists, because it talks about life and sacrifice, it imagines a world in which clones have been made for organ donation; and yet I cannot help but think that this risks missing the point.  The temptation is to assume that Kathy is just like any human, when indeed she is not, and Ishiguro stunningly imagines a type of approximation of humanity, hinting of a substantial difference and of a substantial similarity.  The approximation of childhood and these approximations of relationships are carefully constructed to be chimerical, alien: the only "normals" they encounter are actually sentimentalists who think of these clones as human.