The Postilion by S.M. LaViolette

Duchess Benedicta de Montfort stands to inherit a fortune … if her guardian doesn’t kill her first.
The only way to avoid whatever the unscrupulous man has planned is to hide until Benna comes of age. With the help of a faithful family retainer, Benna becomes Ben Piddock, a postilion seeking employment. Benna knows horses and she’s been mistaken for a tall, skinny boy all her life—so how hard can it be? Very hard, it turns out—especially when she falls in love with her devastatingly handsome employer, Jago Venable, the new Earl of Trebolton.

Jago was working as a country doctor when he inherited his family’s earldom in Cornwall.
Along with the unwanted title is a bankrupt estate that’s a royal mess and a raft of needy, impoverished relatives. One of the few bright spots is his new employee, Ben Piddock, a clever, dedicated, and unexpectedly witty lad–who is no lad, at all! Jago is amazed that Benna manages to hide her delicate beauty from the world while working twice as hard as any man. But as much as he admires and wants her, there’s no way he can pursue a romance with his servant—no matter how bewitching. Yet beyond all reason, and despite his best intentions, he can’t seem to stay away.

It’s clear they were never meant to be together, and yet none of that matters when they’re in each other’s arms. Is there even a possibility of a happily ever after—or will the caprices of fate conspire to keep the two lovers apart?