My personal favorite is Dies the Fire by S.M. Stirling. It's the first of an ongoing series, so if you like it there are almost a dozen more to read afterwards. The book is centered around a small group of people near Portland, Oregon, who band together for survival after a mysterious event (a blinding, painful flash of light that is seen and felt by every human on the planet at the same time) wipes out all technology on the planet. Planes fall out of the sky, combustion engines and even steam power cease to function properly, gunpowder only fizzles and burns slowly instead of igniting in a powerful explosion. In short, the entire planet is thrust back into the Dark Ages overnight, and only those with the foresight to get out of the cities and into the country have any chance of surviving what is eventually known as "The Dying Time" once the food runs out. I'm a sucker for post-apocalyptic fiction, and this book delivers.