S.M. Stirling is a very much hit-or-miss writer in my opinion. He can be pretty good in a pulpy sort of way, and he can also be one of the laziest hacks this side of Turtledove. Dies the Fire is definitely Stirling on one of his better days. It's basically the flip side of his Nantucket novels - the premise being that the same event that flung the island of Nantucket into the 13th century BC also effectively destroyed modern civilization by selectively re-writing the local laws of physics. The extant theory is that "Alien Space Bats decided to take away our toys" - electronics, electricity, explosives, gunpowder, even steam engines all are no longer functional under the new rules. Unsurprisingly, this wrecks humanity on an apocalyptic scale, causing the death by starvation and subsequent disease of at least 90% of humanity within six months of the change.
It sounds like a pretty grim story, and it is in places. But it's hard to get too nasty in a conventional narrative due to the weak anthropic principle - the protagonists have to survive if there's going to be much of a story. In the case of Dies the Fire, said surviving protagonists are various refugees, including a large number of Wiccans, SCAdians, and other hobbyists with the elements of the right sort of experience and training to survive in a world suddenly re-set almost to the 11th Century AD. Stirling is pretty well-known as a fairly conservative, libertarianish sort of writer within the community, and he's wildly unpopular among the left set in fandom for his opinions and his inclination to get into endless arguments in public fora like rec.arts.sf.written. It's rather surprising that he wrote a post-apocalyptic novel about Pacific Northwestern Wiccans and hippies ruling the world, (or at least Oregon) but there it is.
The subject-matter and the setting is very reminiscent of some of Stirling's early stuff, like Snowbrother and the other Fifth millennium books. In fact, I was groping for the comparison, when I realized that I was comparing two of Stirling's novels with each other... I wonder what ever happened to Shirley Meier, anyways?
I still think Stirling wildly overstates just how productive people, no matter how well-motivated, can be in short periods of time. It was a problem with the Nantucket novels, in which our collective heroes had constructed a world-spanning fully-realized civilization inside of ten years, and it's something of a problem with Dies the Fire, which presents trained cavalry, full-scale fortifications, and seige weapony with six months of the event. Civil War era cavalry was supposed to take two years to become well-trained, and that was unarmored, with relatively simple equipment like breechloading carbines. I can't imagine that horseback archers could be *quicker* to train up...
Still, an enjoyable read. I've got the sequel, the Protector's War, on order.