Act Like a Lady, Think Like a Lord by Celeste Connally

Bridgerton meets Agatha Christie in Act Like a Lady, Think Like a Lord, a dazzling first entry in a captivating new Regency-era mystery series with a feminist spin from Celeste Connally.

London, 1815. Lady Petra Forsyth, daughter of the Earl of Holbrook, has made a shocking proclamation. After losing her beloved fiancé in an accident three years earlier, she announces in front of London's loosest lips that she will never marry. A woman of independent means-and rather independent ways-Petra sees no reason to cede her wealth and freedom to any man now that the love of her life is gone. Instead, she plans to continue enjoying the best of society without any expectations.
But when ballroom gossip suggests that a longtime friend has died of a fit due to her "melancholia" while in the care of a questionable physician, Petra vows to use her status to dig deeper-uncovering a private asylum where men pay to have their wives and daughters locked away, or worse. Just as Petra has reason to believe her friend is alive, a shocking murder proves more danger is afoot than she thought. And the more determined Lady Petra becomes in uncovering the truth, the more her own headstrong actions and desire for independence are used against her, putting her own freedom-and possibly her life-in jeopardy.