His Devilish Grace by Lucinda Nelson

The most powerful secrets are the truths you thought you could never reveal… and once spoken, they change everything.
When the scandalous Solomon Dunn, the Duke of Thornehill, discovers that one of his business partners is stealing from him, he knows just the right person to help him find the culprit: Thomas Wolcott, a well-known investigator in London.
Thomas’s sister, Teresa, is not well-liked among the high society of the city, as she speaks her mind and has fits of anxiety at balls and parties. Nonetheless, Solomon is intrigued by her and starts to teach her how to control those fears. Teresa, for her part, enjoys the Duke’s attentions, yet suspects he cannot make a commitment. Can she trust him as she grows closer to him emotionally?
Solomon and Thomas close in on the suspects. But closer seems the mysterious enemy as he is always one step ahead and when word comes out that Teresa is Solomon’s new interest, she will be used as ammunition…
With this endeavour, Solomon will realize that Teresa stole the one thing he thought he never had…His heart.
And just when Teresa finally succeeds to open up this Rogue, she may discover that in gaining love she might lose her life…