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.
Death of an Unsung Hero is the fourth in the Lady Montfort series. It’s $0.99/£0.99 in the US/UK. And discountable at Kobo…
Kindle US: https://www.amazon.com/dp/B074ZJ1KRG
Kobo US: https://www.kobo.com/us/en/ebook/death-of-an-unsung-hero-1
Kindle UK: https://www.amazon.co.uk/dp/B074ZJ1KRG
Kobo UK: https://www.kobo.com/gb/en/ebook/death-of-an-unsung-hero-1