The Three Impostors (published 1895) is an interesting work; while it contains quite a bit of horror, at its core it is a mystery novel, albeit one with ambiguous elements that leave the door open to horror. It features Machen's occult detective Dyson, a somewhat pretentious and sometimes ineffectual presence, but oddly likable. He has pretenses of being a writer but is really an idle dilettante, but often finds himself in weird situations.
The novel is separated into a number of episodes. In the first, "The Adventure of the Gold Tiberius," Dyson, with his Watson (aka Charles Phillipps) find a gold coin in the street, only to discover it is an unbelievably rare and valuable Roman coin. In "The Encounter of the Pavement," they seek the person who dropped the coin, a furtive young man in spectacles, only to encounter someone else looking for the young man, a gent who proceeds to tell "The Novel of Dark Valley." In that, the man tells a tale of being hired as a secretary by a mysterious Englishman who then takes him to America, and then off to an isolated area of Colorado, where he eventually learns his employer is the head of a gang of criminals and is almost lynched when the locals mistake him for his boss. It's actually a fairly forgettable tale; I had to skim it again to remind myself of what went down.
Next is "The Adventure of the Missing Brother," in which Dyson and Phillipps are approached by a young lady who claims to be the sister of the mysterious man in spectacles, but who then proceeds to tell "The Novel of the Black Seal." This is one of the more famous parts of the book, and sometimes anthologized on its own. In it, the woman recounts how she took a job as a secretary to a scholar who lived in the wilds of Wales. The scholar is studying the primitive traditions of the aboriginal peoples of Britain (a repeated theme in Machen), and has the "black seal" of the title, an object that has an inscription in seeming cuneiform, and which is described as "Ixaxar" in an old Roman text, which also explains it as a holy object to a deformed, short people who dwell in the hills. Eventually, his researches lead him to a boy who is a seeming changeling, and then, well, things happen, kind of. It's a tale more shuddersome for what is implied rather than what actually happens, and all the horrors are offstage....still, it's most unsettling and a story that makes a sundrenched countryside seem macabre.
Next up is "The Incident of the Private Bar" where Dyson and Phillipps meet a man who claims to be afraid of the young man in spectacles, and who paints a picture of him as a manipulative con artist. Then comes "The Decorative Imagination" in which Dyson tries to figure out what's going on, and their new friend Burton tells "The Novel of the Iron Maid," a slight tale in which the man tells of visiting a friend who collects torture implements, and witnesses him being accidentally slain by the "Iron Maid" of the title.
Then is "The Recluse of Bayswater" in which they encounter a reclusive young woman, who tells the very, very famous "Novel of the White Powder." She claims to be the sister of a young scholar who was horribly run down, and whose family doctor prescribed a medication in the form of a white powder. At first he seems well, and then behaves oddly, eventually becoming a recluse who never leaves his room. The sister consults the doctor, who analyzes the medication....and horrors, it's not what he prescribed, but something hideously different....
This is the most famous part of the novel, and probably the only Machen story most have read. It actually works well out of the context of the framing story, and has an almost-perfect buildup of horror and suspense, with a satisfying payoff. You will probably find this in any number of anthologies, so check your book collection, you may have it already.
Finally, we have "The Strange Occurrence in Clerkenwell" in which Dyson finally finds the man he's been pursuing, and then learns "The History of the Young Man with Spectacles." He turns out to be a student who fell in with a certain sinister Dr. Lipsius, who runs a secret society dedicated to re-enacting pagan orgies (and, it's vaguely hinted, occult ceremonies and Black Masses). The society wants that coin, and the young man decides to take the coin and clear out. It's then that Dyson and Phillipps learn that all the people they've encountered were impostors, and the tales they've heard were intended only to distract and mislead them.
Finally, in "The Adventure of the Deserted Residence," we have a conclusion that's actually fairly gruesome and tragic. Dyson makes it to the end, but again, he's fairly ineffectual and arrives too late.
It's an interesting variation on Victorian horror, when Decadence was really coming into its own, and free of the "antiquarian" trappings of M. R. James and his imitators, who I love, don't get me wrong, but it's good to see someone of the era going his own way. I've seen Machen classed as one of the "Visionary" school; he prefigures Lovecraft in using the horrors that are just outside human perception, and horror through inference. Lipsius is very decadent, obviously, with his staging of pagan orgies for fun and profit, but Dyson is Decadent, representing the values of spleen and impuissance, and a certain neurasthenia, in his approach to the world. He may be a detective, but he's not a very good one, and seems to be doing it more for the amusement value than out of any sense of right and wrong. That, and his fascination with the weird and uncanny.
Lipsius is also somewhat underused, but he's one of those villains who's best left a cypher. I love his name, though, and once I wanted to use my own version of him as a villain in a horror RPG. I might use him again.
"The Three Impostors" is a mixed bag but is definitely worth reading for those wanting something different from the Victorian age, and to look into a work that influenced many later writers. It's easily available for free, or almost free, online, and there are plenty of used copies out there, like the Ballantine Adult Fantasy edition from 1972 pictured above; that's a lovely addition to your bookshelf.