Arthur Conan Doyle’s A Study in Scarlet, the first book in the Sherlock Holmes series, has to be one of the most adapted pieces of modern literature. Nearly every Holmes’ reboot begins with an updated take on this classic tale, which speaks to the brilliance of Doyle’s writing.

It’s a timeless tale of murder, deceit and the prototypical damaged investigators: the PTSD-stricken Watson and the mentally disquieted Holmes (pick your diagnoses: autism, OCD, bipolar, etc.).

For my money, the greatest adaptation of this story is “A Study in Emerald,” which appears in Shadows Over Baker Street, a mash-up of Sherlock Holmes and H.P. Lovecraft.

Penned by master storyteller Neil Gaiman, “A Study in Emerald” imagines the tale in a post-Lovecraftian landscape, 700 years following the epic struggle between humans and the Great Old Ones.

It shouldn’t come as a shock as to who won that inter-dimensional war, but the nature of the post-war dystopia might. As will the unexpected deviations from the original.

Is it truly unsettling? Not in the same way as most of the books we’ve previously discussed. But it is a bold venture by a gifted author, and the greatest mingling of two of my favorite mythos.

This story also appears in Gaiman’s Fragile Things, along with other favorites like “October in the Chair,” “Bitter Grounds” and “Strange Little Girls.”

For Halloween, Gaiman is offering a free audio book through All Hallows Read. It’s a program that promotes literacy by encouraging people to give someone a book for Halloween. Now through Oct. 31, in partnership with Audible.com, Gaiman is offering a free audio story of his, “Click-Clack the Rattle Bag.” Get yours at www.audible.com/ScareUs.