Highland Wrath by Madeline Martin

After years of training under the mysterious woman she knew only as My Lady, Sylvi is no closer to finding the man with half an ear that brought about the murder of everyone in her family. Now a skilled assassin guiding a network of spies of her own, she has every resource she could want. She just hasn’t found her mark.
On a new assignment, Sylvi’s blade misses its target, but for a change, she is grateful. Her subject, Ian, isn’t just uncommonly skilled and roguishly handsome—he may know exactly where her family’s murderers are.

Now unlikely allies, Sylvi will have to keep Ian close enough to get the information she needs. So close, that she fails to protect her own vulnerable heart. As she and Ian become lovers, she wonders: is a trustworthy ally? Or will he be her downfall?