Stephenson: automatic buy. Even though I don't read a lot of science fiction any more.
Here's sentence number one (so it's not really a spoiler): "The Moon blew up without warning and for no apparent reason." Whoa.
This happens in the slightly-near future: near enough so that the International Space Station is still in operation, but far enough so that we've figured out how to grab an asteroid, and attach it to the ISS for futher study. That's extremely fortuitous, because the Moon's destruction turns out to be, like war, not healthy for children and other living things. Having a big rock around helps.
The book is neatly divided in three: the first part deals with mankind's realization that the World As We Know It is ending, and most normal pursuits become irrelevant. The entire planet's resources and efforts are devoted to ensuring the survival of at least a fragment of humanity. This is not without controversy and squabble. Warning: Certain people are not to be trusted.
Part two deals with the aftermath of the end of the world. The aforementioned squabbles continue, but they become even more deadly, as the spacefaring survivors can't even agree on their short-term survival strategy. Bickering leads to disaster and tragedy.
And then, part three: set 5,000 years in the future. (Just as a benchmark, 5,000 years ago, humanity was just getting around to building the Great Pyramids and Stonehenge.) Things have changed a lot (although I won't spoil the details). But millennia-old conflicts still have their echoes, and they play out in surprising ways.
I very much enjoyed the book. Stephenson is endlessly imaginative, and (I assume) his science is impeccably hard. Parts of the text could be assigned to advanced undergrad courses in Orbital Mechanics, Aerospace Engineering, or Reproductive Biology. As in his previous books, Stephenson's heroes are competent, resourceful, perhaps a little geeky, and brave. I was simply in awe of his talent, all the way through.