What the Devil Knows is the 16th Sebastian St. Cyr mystery by C. S. Harris. Due out 6th April 2021 from Penguin Random House on their Berkley imprint, it's 336 pages and will be available in hardcover, audio, and ebook formats. It's worth noting that the ebook format has a handy interactive table of contents as well as interactive links. I've really become enamored of ebooks with interactive formats lately.
This is consistently one of my very favorite ongoing historical mystery series and always go directly to the top of my TBR list when they come out. The author is adept at writing tightly plotted intelligent mysteries full of pathos and interesting characters from all social strata of Regency London (and environs). All the action occurs against a backdrop of real and fictional political machinations and corrupt dealings with consistently believable and well engineered denouements and resolutions.
I love seeing Hero, her husband Sebastian, manservant Calhoun, and even their carriage tiger, Tom interacting and growing together along with the newest scion of the family tree, young Simon. Although it's the latest of 16 books in the series, it works quite well as a standalone and new readers won't have any trouble following the story. There are developments over the length of the series, so they do read better in order; but it's not absolutely necessary.
Five stars, a worthy addition.
Disclosure: I received an ARC at no cost from the author/publisher for review purposes.