Prince of Darkness is the fourth in the far-too-short four-book Justin de Quincy series by Sharon Kay Penman, who is (much) better known for her straight (i.e. not mystery) historical fiction. I adore this series, and wish there were more, although Penman, who is pretty active on social media, and one of the few authors I follow, has indicated that if she did another de Quincy title it would be a labor of love, because her regular historical fiction sells/pays a lot better. This is series is up in my top ten, probably even my top five, so I keep hoping...
In any case,
Prince has dropped to $1.99 at Kindle and Kobo US, and it's pretty rare for her titles to go on sale, so if you think you might be interested, I'd grab it. I'm trying to think back whether you need to have read the first 3 titles to enjoy this one, but I have re-read these enough that it is hard for me to decide - sorry!
Kindle US:
https://www.amazon.com/Prince-Darkne...dp/B001JJWIAG/
Kobo US:
https://www.kobo.com/us/en/ebook/prince-of-darkness-3