Decoy by Camille Peters

Blair is a prisoner sentenced to die, but on the morning of her execution, she’s spared the noose by an unusual opportunity: pose as a decoy for the princess and learn which member of the court is the secret assassin after Her Highness’s life. If she uncovers the truth before they strike, she’ll receive a full pardon. But if she doesn’t, she’ll be forced to discover the threat at the cost of her life.
Despite the chance to avoid the gallows, Blair has no interest in helping the royal family…until she realizes that this will be an opportunity to complete the mission she was condemned to die for. But the task proves far more difficult than she anticipated when she finds herself entangled with a handsome and charming man—son of a prestigious dukedom by day, heir to the most infamous assassin house by night.
Luke’s involvement with the intriguing princess is solely for the sake of his own mission, and despite the dangers of getting involved, Blair finds herself captivated by the infuriating lord beyond investigating him as her main suspect. Yet target and enemy quickly find themselves drawn to one another by more than the intrigue surrounding them. With the secrets they both keep, falling in love is far too risky…especially with an unknown assailant and a mysterious curse lurking in the shadows, waiting to strike.