Ten Days with a Duke by Erica Ridley

Olive Harper’s family has been feuding with the Westons for decades. The Westons’ stud farm is the biggest, but the Harpers’ is the most famous… and she’s the sole heiress. Or was, until her father brokers a truce by offering the Weston heir the Harper farm. The only way to get it back is to marry the knave who kissed her and humiliated her, twice—or prove to her father that some rifts can never be healed.
Scholar and botanist Elijah Weston is dreadful at feuding. For one, he prefers horticulture to horses. For two, he’s been desperately in love with his mortal enemy ever since he kissed her—and, yes, publicly destroyed her—all those years ago. When he’s given ten days to win Olive’s heart, he arrives with marriage license in hand. But where lies and double-crosses abound, how can lifelong rivals learn to trust their hearts?