This is a post-apocalypse fantasy. It makes a faint attempt at being science fiction by starting in the present day, when a mysterious Change comes and makes essentially all modern technology stop working, instantanously, everywhere, without warning. Unfortunately, other than a few bits of speculation on the part of the characters caught up in the mess, we don't get any explanation of what caused the Change, or what really happened to the laws of physics, and to try to think about it too much will only ruin the suspension of disbelief. So we'll think of it as a fantasy and go from there.
I don't think I'm really violating my no-spoilers rule to say that things start out very bad on the night of the Change and get worse from there. In a couple of particulars I think Stirling is overly grim, but in the overall picture, I think he's excessively optimistic. Almost all of the people are more organized and more effective in the first year after the Change than I find wholly credible. But the story hold together.
The narrative isn't about the overall structure of society, it's about the adventures of a few specific people. These characters are drawn well enough to be sympathetic and interesting, and their adventures are as exciting and as believable as we'd look for in a sword and sorcery novel. I was quite drawn into the world and the characters. At the end of the book, we've reached some partial conclusions, but we're only a year post Change. We're left with reason to hope that things will work out well in the long run, but the main characters and all the rest of the survivors are definitely still in Interesting Times, and there are big issues on the horizon. Not a bad ending all in all: there could be another book or books, but they're not necessary.
Like any halfway rational post-apocalypse story, this book shows us that life without our modern technology would be nasty, brutish, and short. With a lot of luck, it might be a little less so than it was when our ancestors last lived that way. I couldn't help but think about how useless I would be in such a future. It's not very comforting, and the extreme unbelievability of the Change in this story doesn't make it more comforting, because it's all too easy to imagine us ending up in a collapse not quite as quick, but almost as complete. Maybe those thoughts are Stirling's intent, and maybe he just wanted to write a good adventure.
A good book, gripping and even though provoking, but it needs more explanation of the Change than "*poof* technology doesn't work any more -- now what?", and perhaps a better resolution of the big picture at the end of the book, to be great. 8 out of 10.