Lucas by Elina Emerald

Lucas Cambridge is the Marquess of Winchester and eldest son of the Duke and Duchess of Bolton. He fought in the Peninsular War and secretly works for the British Home Office. He is cynical, jaded, and a devilish Rake with a capital R. Marriage is the furthest thing from his mind. Unfortunately, his meddlesome family has other plans. While attempting to extricate himself from one of their ridiculous schemes, he dives behind a curtain during a ball in the hope of escape. The only problem is his hiding spot is already occupied by a most peculiar woman. Something about her piques his interest because she looks vaguely familiar. He has seen her eyes before in a very different setting, and like a moth to a flame, Lucas is determined to find out everything about her.
Caitlin Drummond dreams of the day when her stepmother stops parading her in front of every eligible bachelor within a ten-mile radius. Fortunately, she knows the best hiding spots in every ballroom throughout the ton. A wallflower entering her third season on the marriage mart, Caitlin has no interest in capturing the attention of any lord and views them all with a generous amount of disdain. As the secret apprentice to a progressive physician, she knows the vices most upperclassmen partake of when they are not in polite society. Caitlin is relieved when Lucas Cambridge fails to recognize her and is committed to keeping her distance. Because if he remembers her from that night months ago, it would ruin her life.