This is listed by Goodreads as the sixth book in this series, but it takes place first chronologically, and Amazon describes it as a prequel to the rest of the series. I happened to read it first, and since many of the books overlap, I think it worked out perfectly in that regard. Jane Ashford is usually good with me as regard to premise, but a bit lacking in the chemistry/dialogue, and I wouldn't say this series as a whole breaks that pattern.
Alan is the youngest son of a duke, the academic one, who still lives in Oxford and researches. He's been asked by the Prince Regent, who has some sort of relationship with Alan's dad, the Duke, to clear up a situation involving appearance by the "ghost" of an actress at Carlton House, the Regent's town residence. The Regent wants this cleared up, as he considers that it makes him appear foolish.
The actress in question happens to be the heroine Ariel's mother, Bess Harding, who died by suicide. Ariel is also investigating these appearances, because she doesn't understand her mother's death at all. Of course, they end up combining forces, in more ways than one.
Ariel is supposed to be the fanciful foil to Alan's rather stodgy persona--another review references her as a Regency Manic Pixie Dream Girl, and that's not too far off. To that end, Ariel comes across as a rather sympathetic but immature (in the sense of being girlish) character, while Alan is misogynistic and rigid. I honestly didn't love either one of them, and both of them seemed attracted to the other rather mysteriously and in spite of themselves. I didn't feel any chemistry or buy into their romance.
I did think the portrayal of Bess's mental health issues was sensitive, realistic, and well-done. The ending, however, where Ariel is literally and figuratively legitimized, was a bit out of left field but fine enough.