My personal favorites are Empire of the East, and the Legion of Videssos. Videssos is about part of a Roman legion being transported to another world in which Magic is far stronger than in ours. It was written Harry Turtledove, and the world they are transported to bears a lot of similiarities to the eastern roman empire after the split, except with magic. Turtledove majored or minored in Byzantine studies, so he knows a lot about the period. Empire of the East is a future world (maybe ours) that is devastated by a near nuclear war in which a few bombs go off. Somehow this makes magic stronger, and wizards arise thousands of years afterward. I think it was what the Book of Swords was based on.