Outlander by Diana Gabaldon
In 1945 Scotland, Claire Randall is a nurse, home from the war, and on a sort of second honeymoon with her husband. But while visiting an old stone circle, she is transported back in time to 1743, where she meets Jamie Fraser, a young Scottish warrior, and soon becomes torn between two very different centuries, and two very different men.
Outlander is one of my all time favorite series of books. It is very much historical fiction, but I love the slight Sci-Fi edge, since I'm such a sucker for time travel. But really, it's so well written, I forget about it being even slightly in the Sci-Fi realm. Just a warning though, there are some pretty graphic scenes, of both violent and sexual natures.
I give this book an A.
The next book in the eight book Outlander series is Dragonfly in Amber.