Dies the Fire by S. M. Stirling - The Change, a mysterious event, renders all electricity and explosives useless. From this apocalyptic event rise the survivors, an eclectic group of folks who start a new society based on concepts from the Middle Ages. But there are other survivors, those who are bent on conquering the world and forming it in their own twisted image.
This is a pretty straightforward apocalyptic tale. I enjoyed it though. At times, it did drag and the whole Wiccan community/religion thing got on my nerves a time or two, but all in all it was worth the read.