It's in the Duke's Kiss

Lady Emmaline Radcliffe is determined to avoid marrying a conventional gentleman, but pressure from her mother to find a suitable match forces her to reconsider her youthful crush. When she finds herself entangled with the serious Duke of Blackbourne, she begins to question her desires. As Lucian, the Duke, attempts to protect them both from his reckless brother's advances, he discovers an unexpected attraction to the very woman he thought he didn’t want. With his brother's persistence complicating matters, Lucian must show Emma that he’s more than the uptight duke he appears to be.