This is a post-apocalypse story, which is a scenario I particularly like, partly as it often drops hints so you can work out what famous landmarks have become, or what a culture has become. For instance in this book I'm pretty sure one of the enclaves is what the Church of Jesus Christ and the Latter Day Saints are in this vision of many thousands of years in our future.
I also like post-apocalypse stories because they can give you a good idea of what the author thinks are the disasters waiting to happen in our current society. This story, in common with some other recent books I've read, postulates nano-tech as being involved in the destruction of society as we know it. If you look back at other times other concerns are apparent - most new books that I read in the late eighties and early/mid nineties postulated some version of the greenhouse effect, or other runaway changes in the ecosystem of the earth, as being the doom that faces us. Looking further back, earlier in teh eighties and also in the late forties and early fifties it was nuclear war and resulting aftermath that would finish off civilisation. Those two eras seem to differ in the level of knowledge of the problems to be faced, although I've read 'mutants everywhere' stories from the eighties, and 'dying of radiation' stories from the late forties. This is all based on my 'feel' of the things I've read. Some day I'd like to sit down and systematically go through, reading post-apocalypse stories and correlate year/era with disaster faced. It'd be interesting :)