This was one of those occasional books picked up rather at random and/or on the strength of its cover and blurb. Yes, this occurred in time-honoured fashion in St. Pancras railway station in the new Hatchards. I had never heard of it, yet something about it appealed. Good, because I am deeply glad I read this book, even if, even now, I'm not quite sure I know what I make of it. I have a sneaking suspicion I didn't get the half of it, somehow skating superficially over the surface of a massively deep book.
It is fundamentally a story of sibling love and loss, of what it means to be human, of what a family is and should be, of what it means to belong to a society, of the value of life itself - not just human life. There is a deep thread of guilt too running through this book - how to live with the sense of something which went drastically wrong, though quite how and why remains elusive. (Shades in more than one way of Behind the Scenes in the Museum here.)
So far, you might also think, so not-very-unusual for modern literary fiction. Until, in one of the biggest twists I have come across, you discover, somewhere between a quarter and a third of the way though the book that...
NOTE: it is impossible to talk meaningfully about this book without "spoiling" the huge twist referred to above. It doesn't really change much, I suspect, but if you want, as I was, to be gobsmacked by the revelation, stop here, do not read on.