Persuade A Highlander by Rebecca Preston

When Cara travels to Dunscaith to investigate a Celtic myth and accidentally finds herself flung back in time to ancient Scotland, she must face her growing feelings for William MacLeod while investigating the very mystery she sought…
Anthropologist Cara Eastman arrives on the Isle of Skye to investigate Dunscaith’s connection to a series of myths around a woman warrior—the Scathach. After stepping through the unbarred door to the past, she finds herself in the right place at the right time, since the very thing she intended to investigate has made a reappearance. Only, this time, the Scathach is leaving a trial of bodies in her wake.
Carousing, troublemaking guardsman William MacLeod accepts the dangerous take of investigating the deaths as a way to prove himself to the Laird. When he finds his peers unable or unwilling to assist, he turns to Cara and her unique insight into the Scathach in order to get a leg up.
Neither expects to fall in love, but when the mystery deepens and it appears an imposter is responsible for the murders, they must trust their new bond is enough to save them from becoming the next victims.