You can also use these as alternatives to actually reading the Outlander books, if you love the TV show but also don't want to directly give Diana Gabaldon your time and money, given the way she bites the romance-reading hand that feeds her.
All three of the recommendations here are rich in history, setting, characterisation and plot. Two are set - or partially set - in Scotland, while the last incorporates the time-travel element but has an enticingly different setting.
2. Highland Rebel by Judith James
Highland Rebel is set during the Glorious Revolution when the Stuart King James II was deposed in favour of his Protestant daughter Mary and her husband William. While campaigning in Scotland, English spy Jamie Sinclair marries Highland lass Catherine Drummond to keep her safe. As the Revolution unfolds, Jamie and Cat must navigate shifting political and religious alliances, as well as the finer points of their marriage. Jamie can be a bit of an alpha-hole at times, but it has the same saga-like feeling as Outlander, as well as the Scottish and Stuart elements.
3. Beautiful Wreck by Larissa Brown
Since the previous two have adhered pretty closely to Outlander's Scottish setting, Beautiful Wreck is a bit of an outlier. But it has time-travel and a very Gothic, slow-burn vibe that I find very reminiscent of the first season of the TV show. It's set between the 22nd century, and 10th century Iceland, with the heroine being thrown back in time as she tests a machine that simulates the past. Brown conveys the harsh life and inhospitable environment faced by the early Icelandic settlers extremely well, as well as the sense of adapting to a new life.
If anyone else has some more suggestions for Outlander-esque romances, hit me up! I'd love to hear from you.