A great series of books is:
The Videssos Cycle by Harry Turtledove. There are 4 books in the series, the first being:
The Misplaced Legion
As they faced one another in a duel of survival, the Roman tribune Marcus Scaurus held the spell-scribed sword of a Druid priest, and the Celtic chieftain Viridovix held a similar sword, bespelled by a rival Druid sorcerer. At the moment they touched, the two found themselves under a strange night sky where no stars were familiar and where Gaul and Rome were unknown. They were in an outpost of the embattled Empire of Videssos--in a world where magic and dark sorcery would test their skill and courage as no Roman legion had ever been tested before.
Book #2 is called:
An Emperor for the Legion
Book #3 is called:
The Legion of Videssos
Book #4 is called:
Swords of the Legion
