Outlander by Diana Gabaldon #1

The year is 1945. Claire Randall, a former combat nurse, is just back from the war and reunited with her husband on a second honeymoon when she walks through a standing stone in one of the ancient circles that dot the British Isles. Suddenly she is a Sassenach—an “outlander”—in a Scotland torn by war and raiding border clans in the year of Our Lord . . . 1743.
 
Hurled back in time by forces she cannot understand, Claire is catapulted into the intrigues of lairds and spies that may threaten her life, and shatter her heart. For here James Fraser, a gallant young Scots warrior, shows her a love so absolute that Claire becomes a woman torn between fidelity and desire—and between two vastly different men in two irreconcilable lives.