This is book number 5 in the True Gentlemen series.
Jonathan Tresham, heir to the Duke of Quimbey, needs a discreet ally to help him choose a wife from among the army of young ladies eager to become his duchess. When proper widow Theodosia Haviland rescues him from a compromising situation, he knows he's found an advisor he can trust. Theo's first marriage taught her the folly of indulging in romantic notions, and she's determined that Mr. Tresham's intended be an ideal dynastic match for him, not some smitten ninnyhammer.
When Jonathan decides that Theo's is the only name on his list of possible duchesses, she protests, though she knows that Jonathan is kind and honorable despite his gruff exterior. Theo is guarding secrets, and a ducal heir is the last person she can marry, even if she does harbor an entirely inappropriate attraction to him.