Smoke and Jewel by Kate Grove

A fugitive ninja. A kind-hearted witch. 16th-century Japan. Traitors don't deserve a happy ending, do they?

Karen has finally found a clue to her cousin Ciara's disappearance. But before she can follow, she's kidnapped and finds herself in a war-torn, 16th-century Japan. The madman that kidnapped her forces Karen to use her magic to enchant weapons in a castle full of half-yokai. To top it off, her magic is restricted. There's no escape… unless she trusts an untrustworthy ninja, Taiki.

Taiki is an excellent spy and assassin. He's also a traitor, as Karen learns.

If he's willing to betray someone as powerful as Katsuo, what would he do to her? Which is worse; trusting a traitor or facing a bleak life as a slave enchanting weapons? To complicate matters further, she's falling for the aloof ninja even as he struggles between saving her life at the peril of his own. If he helps her—and is caught—he risks certain death. Will her heart survive any outcome?

Smoke and Jewel is the second book of the Youkai Treasures series, a fantasy romance set in the sengoku-jidai (16th-century Japan).