I was talking to a friend of mine tonight about some of my favorite alternative history books, so I thought I would list a few here:
Rally Cry by William Forstchen – This is the first of an eight book series about a Civil War Union Regiment of Engineers that are transported to an alternative Earth, where they have to build up their technology and industrial base in a few years to avoid being eaten by nomadic aliens.
The Cross Time Engineer by Leo Frankowski – This is the first of a seven book series about an electrical engineer who gets thrown back in time to 13th century Poland, a few years before the Mongols are due to invade from the East. Can he use his technical skill to help build up the technology and military level of Poland without being burned at the stake for being a wizard?
Guns of the South by Harry Turtledove – What would happen if racist time travelers from South Africa went back to 1862 with blueprints for the AK-47 for Robert E. Lee and the Confederacy?
Dies the Fire by S.M. Stirling – If the laws of physics changed, rendering electronics, electricity, explosives, firearms,and internal combustion inoperable, what would happen to modern civilization? Well first, a lot of people would starve to death pretty quickly. But after that, how would the survivors try to rebuild?