Join A Highlander by Rebecca Preston

When Sarah chases a thief through a door that thrusts her back in time to ancient Scotland, she must hunt down not only her quarry but also those stirring up trouble for the MacLeods, including the handsome and honorable Kieran who may bring trouble to her heart…
Hardboiled detective Sarah Elway always gets her man, but when she follows a diamond thief through a door in Dunscaith Castle on the Isle of Skye, she ends up in ancient Scotland and in the middle of troublesome bandit attacks.
Already ragged and on-edge due to the unrest in the area, Captain of the guard Kieran MacLeod feels obligated to keep the troubling woman from the future who appeared in the castle safe—not only from the bandits attacking but also from her own reckless behavior. He isn’t used to dealing with someone so strong or independent, and Sarah refuses to listen to him or his warnings, which leads to heated arguments and even hotter attraction.
When those Kieran is hunting team up with the man Sarah chased through time, they find themselves working together to fight a common enemy that threatens the entire clan. But Sarah’s stubbornness and Kieran’s honorable nature stresses their new love and forces them to face hard truths in order to keep themselves and everyone else safe.