Merely a Marriage
Merely a Marriage is her 41st and "final" book -- it was completed shortly before she died, and published posthumously on 30th May 2017 by Berkley/Jove .
The e-book was published simultaneously.
More info is available here:
As England mourns the death of Princess Charlotte, Lady Ariana Boxstall has another succession in mind. Her brother, Norris, is a strapping young man, but he's also happily unmarried and childless. Norris agrees to take a wife on one condition: that Ariana take a husband first. Although she realizes she risks a lifetime in a loveless marriage, for the sake of her family, Ariana accepts his challenge.