If you liked Outlander by Diana Gabaldon, you might enjoy one of these books:
Devil Water by Anya Seton
Seton wrote several thick books with elements of appeal for Outlander fans—though no one book has everything. They are nicely paced, involving stories woven through with solid historic detail and attention to character. Here, Jenny, the daughter of the last Englishman beheaded for supporting the pretender James Stuart in the Jacobite Rebellion, goes to Virginia.
Into the Wilderness by Sarah Donati
Donati regards Gabaldon as her mentor; her work also has solid history, evocative settings, love between "outsiders," and strong characterization. In the first book of her saga, Elizabeth leaves England to teach in New York. When she meets Nathaniel, a white man raised as a member of the Mohawk tribe, their love causes a scandal that leads them to flee into the woods and into danger.
Ivanhoe by Sir Walter Scott
For early historical romance, try the classic Ivanhoe. At the time of the Norman Conquest, Ivanhoe returns from the Crusades to claim his inheritance and the love of Rowena. He becomes involved in the struggle between Richard Coeur de Lion and his Norman brother John. The gripping narrative is structured by a series of conflicts: Saxon versus Norman, Christian versus Jew, and men versus women.
A Knight in Shining Armor by Jude Deveraux
Abandoned by her lover after a quarrel, Dougless Montgomery is stranded in rural England. The sudden appearance of Nicholas Stafford—a knight who died in 1564—begins a passionate affair that eventually takes Dougless back to the 16th century. Also try Legend, in which an antique wedding dress leads to 1873 Colorado.
Kushiel’s Dart by Jacqueline Carey
Sold into indentured servitude at the exotic Night Court as a child, Phèdre nó Delaunay faces a difficult choice between honor and duty as she deals with a world of conspiracy and betrayal. Adventure, action, romance, and steamy sex figure prominently in this sprawling, intricately plotted fantasy series.
Lord John and the Private Matter by Diana Gabaldon
John Grey, who befriends Jamie Fraser at Adsmuir prison in Voyager, has his own series set in the Outlander world. Here, upon his return from Scottish exile in 1757, Lord John pursues a traitor through London and across the seas, an endeavor that is complicated by a delicate family affair and his memories of the Jacobite Rising.
The Pillars of the Earth by Ken Follett
Readers who enjoy Gabaldon’s detailed stories of everyday lives during extraordinary historical times should try this modern classic. Set in 12th century England, this epic of kings and peasants follows the lives, loves, dreams, and heartbreaks of Kingsbridge during the construction of a magnificent cathedral.
Quicksilver by Neal Stephenson
Set against the backdrop of the late 17th and early 18th centuries, Quicksilver tells the intertwining tales of three unforgettable main characters as they traverse a landscape populated by mad alchemists, Barbary pirates, and bawdy courtiers, as well as historical figures including Ben Franklin, William of Orange, Louis XIV, and many others.
The Skystone by Jack Whyte
A retired centurion and his former commander conquer enemies and establish an independent community in Britain, while also providing a possible origin for one of the greatest artifacts of Arthurian myth—Excalibur. "From the building blocks of history and the mortar of reality, Jack Whyte has built Arthur's world and showed us the bone beneath the flesh of legend," Gabaldon has written of this series.
Son of the Morning by Linda Howard
In this beloved time travel romance, ancient language translator Grace St. John finds a document that gives the location of a treasure that Niall of Scotland brought home as Guardian of the Treasure for the Knights Templar in the 14th century. She is transported to his time to protect the relics, including the Holy Grail. Niall is one sexy Scot in a class with Jamie Fraser!
Timeline by Michael Crichton
When elderly Yale history prof Edward Johnston travels back to his beloved 15th century and gets stuck, his assistants follow to the rescue. Crichton invests his story with terrific period detail, castles, sword-play, jousts, sudden death, bold knights-in-armor, and seductive ladies-in-waiting. There's also strong suspense as he cuts between past and present, where the time-travel machinery has broken. Will the heroes survive and make it back?
Time and Again by Jack Finney
Simon Morley, a young Manhattan illustrator, is selected by a secret government agency—presumably to test Einstein's theory that the past actually co-exists with the present—and finds himself transported to the year 1882 under hypnosis. There, he falls in love and refuses to change records for the government agency controlling his experiment. Written with style and elegance, this novel is boldly visionary yet romantic.
The Time Traveler’s Wife by Audrey Niffenegger
For a much different twist on time travel romance, try the story of Henry and Clare Detamble. Henry’s Chrono-Displacement Disorder forces him to travel against his will, causing him to visit Clare as a little girl and later as an aged widow and explain "how it feels to be living outside of the time constraints most humans are subject to." This novel is a beautiful, heartbreaking, timeless love story.