Duke of Luhst by Mariah Stone

Nothing could change him for the better. Save, perhaps, her.
Seductive as a deadly sin, Lucien Wrenn, the Duke of Luhst, can tempt any lady into his bed. But his best friend’s little sister—bluestocking scientist Lady Chasity Perrin—is forbidden fruit. And he’s spent a lifetime aching to take a bite.

Chastity has always loved Lucien but the thoughtful boy she cherished grew into a careless rogue. Content with her lot as a spinster, she fights to advance her medical research in a world that denies women’s place in science.
They can never be together until they clash in a heated argument and make a scandalous wager… During a month-long house party, both must do the impossible: sinful Lucien must remain celibate and wallflower Chastity must secure a marriage proposal.

When every eligible bachelor begins to compete for made-over Chastity, Lucien burns to claim her as his own.
As summer storm traps them, no vow of friendship can hold him back from breaking every rule to possess her.

As they grow closer — body and soul — his touch unravels her and she begins to crave something very dangerous: him. For the first time, someone sees straight to his wounded soul, and makes him want to be worthy of the only woman he loved his whole life.