Death of a Dishonorable Gentleman is the first in the Lady Montfort series by Tessa Arlen. Per SYKM, it was a finalist for the 2015 Agatha Award for Best Historical Novel. It has dropped to $1.99 at Kindle and Kobo US. I haven’t read any books in this series, but have read and enjoyed both of Arlen’s later Poppy Redfern books.
Death of a Dishonorable Gentleman is $1.99 again at Kindle US, but sadly, this time, not on sale at Kobo US. The link in the OP just above is still good.