Post-apocalypse Dies the Fire by S.M. Stirling. Very questionable physics, courtesy of the Alien Space Bats, suddenly kiboshes gunpowder, high-pressure steam engines, all electronics, and internal combustion gasoline engines. The world devolves immediately into riots, plague, and the ascendancy of coincidental cool ex-military, and Creative Anachronism or Wiccan types who were largely shunned as geeks in the old real world. Seems creepily like major wish-fulfillment on the part of the author?
To be fair, the book is quite entertaining and plausibly, solidly plotted and detailed. Give it a try.