Outlander series by Diana Gabaldon

I stumbled upon this book through book club. It's the first in a series of 8 books. Claire Beauchamp Randall is a nurse and wife to Frank Randall. During WWII, she was a nurse in London and he had some sketchy assignment at MI-6 although he is a history professor by trade. After the war, they go to Scotland for a second honeymoon. They've been separated for 6 years and would like to get re-acquainted. Meanwhile, Frank is tracing his ancestors and finds that one of them, Captain Johnathan "Black Jack" Randall, a Redcoat during the Jacobite uprising against King George was stationed in the Scottish Highlands.

On Beltane (Spring Equinox) Claire visits the Henge on Craigh na Dun to pick flowers. She's an amateur botanist and likes to dabble in herbal remedies. She notices one of the giant rocks has a split in it and is buzzing. When she touches the rock, the rock screams and she's pulled into chaos. When the noise dies down, the screaming continues and she sees a band of men in the midst of an historical re-enactment of a battle between Redcoats and Scottish Clansman. She runs into the woods to get out of the way and Frank, dressed as a Redcoat, grabs her...except upon closer inspection, that man isn't Frank. He introduces himself as Captain Johnathan Randall!

Claire is soon rescued from Black Jack's unwanted advances by equally unwanted Clansman and taken to the MacKenzie clan and along the way, she befriends Jamie, a lad of mere 23 who she has to cobble back together after a few battles. She slowly accepts that she's time-jumped and slowly accepts that Jamie, although 5 years her junior, may be just what she needs.

Without spoiling it, Claire and Jamie withstand a lot of danger, heart-ache, and love. But when the time comes, will she go back to Frank and her own time? Or will she stay in 1743 with Jamie?