The Outlander series by Diana Gabaldon is outstanding. It's hard to categorize it because it has elements of so many things. The premise is that Claire, a British nurse who has just finished tending to soldiers during WWII, has reunited with her husband Frank who was an officer during the war. They head to the Scotland Highlands for a second honeymoon and while there go to a stong ring (something like Stonehenge) where Claire ends up getting sent back in time to the seventeenth century. There she is almost immediately thrown into a situation where her nursing skills are called into use. I won't say anymore about the plot because I don't want to give away any major plot points.

One caveat: These books are most definitely rated R. There are some pretty graphic descriptions of war, torture, rape, sex, and just plain gruesome living conditions.