All That Glitters. Lord Alchemist, #4

To ensure his sons' immunity to hostile potions, the Lord Alchemist defied tradition to marry the only immune woman for generations: a half-barbarian. Jani's been hired to blackmail the Lord Alchemist into disinheriting his "mongrel" heir, Iontho. When she mistakes Iontho for a servant, Iontho takes the opportunity to pose as her alchemically-loyal minion and seek information about her mysterious employer.
While the travel is worse than he'd expected, the company's better than either had hoped, and young hormones are fully engaged - but can youth and unexpected skills defeat age and treachery?(All That Glitters is an approximately 92, 000-word novel, and has mature scenes.)