An alternate universe of Sherlock Holmes canon. I have read it before but I saw the audio version on Audible and decided, eh, why not?
This particular version was narrated by Neil Gaiman (I thought it was Gy-man, not Gae-men? Huh, go figure.) and he has a lovely voice, but it irritated me profoundly that he pronounced Lestrade as Le-stray-d. Just... no. It does not sound nice.
The story, of course, takes place in England, but one renamed Albion and in which supernatural elements are at work and creatures - for lack of a better term - literally rule. The story is narrated by the consulting detective's companion, a veteran of the Afghan war. A strong knowledge of the entire Sherlock Holmes series is practically a requirement due to the reliance on some of the lesser-known canon references used to make the twisty ending come to light. Likewise, the faux contemporary advertisements that break up the chapters require at least a mediocre familiarity with both 12+ grade English vocabulary and classic horror stories.
As evidenced by the name, the main plot shares similarities with Conan Doyle's "A Study in Scarlet." Probably a good place to start as a prequel of sorts for the uninitiated. :)