My Own True Duchess by Grace Burrowes

Jonathan Tresham, heir to the Duke of Quimbey, needs a discreet ally to help him choose a wife from the mob 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 Jonathan Tresham’s intended be an ideal match for him, not some smitten ninnyhammer.
When Jonathan suggests Theo should be at the top of his list of possible duchesses, she protests, though she knows that Jonathan is kind and honorable despite his gruff exterior. The last person Theo can allow Jonathan to marry is a widow guarding scandalous secrets, even if she does also harbor an entirely inappropriate attraction to the one man she can never have.