This is part of that particular offshoot of Holmes pastiches that uses not the Great Detective himself, but his author, Arthur Conan Doyle. This tale penned by Twin Peaks
scribe Mark Frost thrusts a young Conan Doyle into an occult mystery leading to the rise of a dark power, the identity of which provides a bit of a twist at the end. Conan Doyle serves as a sort of Watson figure to a Holmes stand-in who works as a secret agent for the British government. I found it a crackling yarn that tickled my fancy for Victorian occultism.