Outlander & Lord John Series by Diana Gabaldon

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.

The books in suggested reading order
Outlander
Dragonfly in Amber
Voyager
Lord John and the Hand of Devils
Lord John and the Private Matter
Lord John and the Brotherhood of the Blade
The Custom of the Army
The Scottish Prisoner
The Plague of Zombies
Drums of Autumn
The Fiery Cross
A Breath of Snow and Ashes
An Echo in the Bone
Written in my Own Heart's Blood
A Leaf on the Wind of All Hallows
The Space Between
Virgins