I ordered a copy right away and, for reasons I no longer remember, it sat on my shelf unread ever since. In fact, the only reason I picked it up now is because kelliwithani82 reminded me of its existence by telling me she's having her students read a slightly abridged version in class. I figured I'd read it at the same time as her sixteen-year-olds!
My edition of The Great God Pan is quite a lovely one, published by Creation Classics in 1993. It features not one, not two, but three introductory essays, one of which was penned by Machen himself for the 1916 reissue. It also features illustrations by the "mystic" Austin Osman Spare (a contemporary of Machen's) ostensibly in the "automatic" style, which means Spares supposedly entered a trance state and drew the illustrations unconsciously. I doubt that very much, since the illustrations are not cramped, illegible squiggles but actually very fine line drawings. They don't always connect with what's happening in the text, most of them are really just images of satyrs with boobs, but they do match the feel of the story with their decadence and grotesque sexuality.
The story itself concerns an amateur investigation into the mysterious woman Helen Vaughn, who brings misfortune, ruin, madness and death (by shock!) to the lives of anyone unlucky enough to cross her path, as well as the suspicion that the answers may harken back to an experimental attempt, witnessed years ago by one of the protagonists, to lift the veil between this world and that which lies beyond. The revelations will most likely not surprise today's readers the way they did in 1894, when The Great God Pan was first published, but it's easy to see how Machen's tale of cosmic horror strongly influenced the fiction of H.P. Lovecraft. (One can also spot an influence on Peter Straub's Ghost Story, in which characters die of shock upon witnessing the somewhat similar secrets of the mysterious Alma Mobley.)
Machen was a master stylist, but the manner in which he tells his tale may confound modern readers, who are used to their protagonists taking an active role in the story rather than sitting in drawing rooms and discussing their off-page discoveries, as they do here. Still, that was the dominant style of the time, especially for terror tales, and the story is no less affecting because of it.
If, like me, you're one of the last horror enthusiasts on the planet yet to read The Great God Pan, I highly recommend it. It's not just an important piece of genre history, it's also a compelling and chilling story of cosmic terror.