Some people probably read this expecting science fiction or something out of the alternative history genre, what with all the clones running around. It's nothing like that. Today's biological science and genetic engineering potential make clones the perfect vessels for exploring what it means to be human and alive. Ishiguro does that well, and his conclusion is unsettling.
When I say that I don't mean it in an I, Robot sense, or like whatever non-human they had as a walking advertisement for What It Means To Be Human in the various Star Trek series. The novelty of being proud of yourself for recognizing a character in a book, movie or TV show as, ironically!, the most human of them all even though he's not wore off probably after about a year of Mister Data, if not after Asimov got through with it. Ishiguro isn't going for that at all, to his credit. The characters in Never Let Me Go are decidedly not "as human" (let alone more) as biologically born persons are, and that is the profoundly disturbing thing that makes the book haunt you after you close it for the last time.
The rich experience of the book is enhanced the less you know about it in advance, not that there are many spoilers, but chances are you know that in the book clones are grown, kept healthy and brought to adulthood in order to donate various organs and such to non-clones. It doesn't take place in the future; instead, advances in genetic engineering have happened at a far more accelerated rate after World War II than they did.
We follow the conclusion of the career as "carer" to her fellow clones of a Kathy H., during which she reminisces about her childhood and young adulthood. Kathy is a wholly remarkable character who, unlike her fellows, doesn't have lifelong dreams of a "normal" life working in an office or become preoccupied with rumored "deferrals" of her donation obligation. You want her to live a great deal more than she does -- not that she affirmatively wants to die, at all, but part of the brilliance of the development of Kathy is that it is truly, and believably, not in her to think in terms of life or death. She looks forward to being able to "retire" from her duties as a carer, at one point even betraying that she's looking forward to the peace, quiet and slower pace -- of having her organs harvested. She never thinks in the terms illustrated by the part of the previous sentence after the dash, and this is a remarkable accomplishment in a sympathetic character.
You intellectually understand and accept how Kathy's and Tommy's relationship develops and where it's going, but it's passionless (most things are) and you never "root" for them to live happily ever after. You may think that means you don't care about them, but Ishiguro's achievement here is that you do. Without resort to the formulaic buttons authors can push to get you to identify with and like human characters (or Mister Datas or Spocks), Ishiguro interests you in his. Read the book to appreciate an extraordinarily difficult job of character development done well, and you won't be as disappointed as if you're looking for sci fi.