"A long shear of light and then a series of low concussions" - this is how the world ends in The Road - by Cormac McCarthy, 2007 Pulitzer Prize for Fiction winner. The most brilliant and emotional book I've read in years - it literally made me cry.
It is dark and horrific and haunting - and so well written you can't help but be sucked in. As a dad I couldn't help but empathise with the father in the story - I don't think you can understand the pure love he shows for his son (and the boy for his dad) without having a child of your own, a child you truly believe you could and would die to save if necessary.
I have always been fascinated and emotionally affected by post-apocalyptic fiction - movies and written (for example Mad Max, 28 Days, The Postman, Nausicaä of the Valley of the Wind, Waterworld, Star Trek: First Contact and recently the TV series Jericho) which makes this story even more fascinating for me.
It isn't science fiction (you never find out what caused the end of the world, no aliens and little science) and I don't think I would ever have read it myself if it wasn't given to me as a gift. Do yourself a favour and give it a try, it may change the way you see life, the world and everything.
Links: The Road (novel) - wikipedia article (warning: spoiler), Wikipedia's List of apocalyptic and post-apocalyptic fiction, and the NPR book review.
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I think "science fiction" these days is a bit of a misnomer. Peter F. Hamilton's Night's Dawn trilogy has spaceships and some aliens, but light on the science side and has souls coming back from the dead. Categorized more as space opera, if I was being strict, I'd call it a fantasy rather than SF.
The Road, though, is clearly Science Fiction, or rather Speculative Fiction. It is a fictional look at a possible future. SF.
Ray Bradbury once said that the only work of science fiction he wrote was Fahrenheit 451 because the events in that novel could actually happen. Everything else, including The Martian Chronicles, was fantasy. Aliens and science are not requirements for "science" fiction. The genre, such as it is, is much wider than that.
Any piece of post apocalyptic fiction is, by its nature, SF.