Outlander

English nurse Claire Randall and her husband take a second honeymoon to reconnect with one another in the Scottish Highlands in 1945 (they have been separated by 8 years of war). Claire is on a walk alone on an nearby hill when she walks through a cleft stone and is transported to 1743. She is saved from an evil English dragoon by a Highland clan, and befriends one of the Highlanders. What follows is a tale of adventure, espionage, romance, and a woman torn between fidelity and desire, and her torn conscience about returning to her own time and her newfound love in the 18th century.
There are currently six (very long) books, a seventh in the works, a graphic novel based on the first, and a spin off series based on one of the characters who first appears in book two. ~Woogie

The books are (in order):
Outlander
Dragonfly in Amber
Voyager
Drums of Autumn
The Fiery Cross
A Breath of Snow and Ashes
An Echo in the Bone (to be released in 2009)