The Dream Book by Suzanne Ress

Accidentally finding a stranger's dream book lead an American expatriate into a series of misadventures, which eventually results in her being kidnapped and imprisoned by an unusual warrior-like man, in an ancient tower. She is freed two thousand years in the past, still in the same small town in Lombardy, Italy. Here, she is revered as a goddess by the local Celtic population, in part because of her metal-smithing skills, in part because she carries the child of the warrior, Viridomarus.