Now to add my own. I just recently read two Trilogies written by S. M. Stirling. Both of these two series were so good that I went several nights without enough sleep just so I could read "just one more page".
Both trilogies are known as "Novels of the change" but both are told from a different group of people experiencing the change in different ways.
The first trilogy is as follows:
Dies the Fire
A Meeting at Corvalis
The Protectors war
The plot is simple yet complex. One day in 1998, there is a flash that starts in Nantucket and some of the laws of physics stop working the way we know them. This comes to be known as "The Change". Gun powder and Electricity just stop working. These three Novels follow a few groups of survivors in Oregon, who have to revert to medieval farming, fighting and in some cases, Empire building to survive but they do have a lot of our knowledge to help them out. Imagine Castles being built with our current knowledge of construction.
The second Trilogy is as follows:
Island in the Sea of Time
Against the Tide of Years
On the Oceans of Eternity
One day in 1998 the island of Nantucket gets thrown back to about 1300 BC but this time everything is working as usual. The islanders have to revert back to making everything (food, fuel, housing, everything) themselves because there is no more USA and therefor no infrastructure. Imagine what would happen if you had our current technological knowhow and were sent back to the Bronze age and you have the basic plot of the trilogy.
I would highly recommend both Trilogies. If my descriptions seem a bit bare that is because I am specifically avoided giving any spoilers. Lets just say that, after reading both these trilogies, I am going to read everything that S. M. Stirling has ever written.
. In order of preference:
