Oddity of the Ton by Emily Royal

An unlikely pairing becomes the perfect match.
Eleanor Howard has never fitted in. To her, Society is an unfathomable world where success is measured by the number of suitors’ names on her dance card. And her card is always empty. Constantly outshone by her younger sister, and continually criticized by her ambitious mother, Eleanor yearns to be loved for herself, not what others expect her to be. Her secret infatuation with the Duke of Whitcombe—who’d never deign to notice, much less court her—would, if revealed, make her the laughingstock of the ton.

Until he strides across a crowded ballroom and offers his hand.
Montague FitzRoy, fifth Duke of Whitcombe, is unwilling to surrender the pleasures of bachelorhood. When his mother, insistent on finding him a duchess of the right pedigree, forces his hand, he decides on impulse to punish her by kneeling before the least desirable woman in the room.

Only Miss Howard believes his proposal to be genuine.
The solution? A false betrothal that keeps Monty’s mother in check, and lifts Miss Howard’s prospects. Monty can weather a few weeks’ ridicule over an engagement to a woman whose eccentricities are gossiped about. And a betrothal—even short-lived—to a duke will ensure that Miss Howard’s dance card is never empty again.
But beneath Eleanor’s awkward exterior lies a passionate, intelligent woman, misunderstood and unappreciated, who challenges Monty’s notions of duty, life, and love—and with whom he’s falling in love himself.

As the end of the Season, and their inevitable parting, approaches, Monty is faced with a choice—surrender to duty, or follow his heart.