The Great God Pan
What is now the first chapter of the novella was published in 1890 in a magazine called "The Whirlwind", while what is now the third chapter of the book was published in the same magazine the following year as a standalone short story called "The City of Resurrections". Machen only viewed the two works as connected after they were finished. Once he decided the two stories were connected, Machen wrote the rest of "The Great God Pan" in a single evening save for its final chapter. Machen did not think of an ending for the tale for months, and in that time believed that the novella would remain unfinished forever. The last chapter was completed in June 1891. Machen sent the novella to the publisher Blackwood, who rejected it, deeming it a clever story that "shrink[s]...from the central idea." It was accepted by John Lane and published in 1894. When published as a book, "The Great God Pan" was accompanied by another Machen tale called "The Inmost Light" which also features a mad scientist and elements of science fiction. The book's cover was illustrated by Aubrey Beardsley.
Spider (pulp fiction)
At roughly the same time in England, Mews Books/New American Library reprinted four Spider novels sporting new cover artwork, each different in style and execution from those used by Pocket Books. This Spider mass market series also ended after only four titles had been published. 
Then, three years later, in 1979, an unusual Spider publishing event happened right "out of the blue". Python Publishing put into print the never-before-published last original Spider novel, "Slaughter, Inc." (written by Donald G. Cormack), originally to have been published as "The Spider" pulp magazine #119. Python published it as a one-shot mass market paperback. For copyright reasons all character names were changed, and the novel was retitled "Blue Steel" ("The Ultimate Answer To Evil") and the author listed as "Spider Page" (no doubt a sly reference to the novel's origins as a Spider novel and the character's principle author, Norvell Page). The Spider was recast as the title character, Blue Steel. As with Pocket Books' Spider editions, this paperback sported a "modern" pulp cover painting featuring a very similar, non-costumed, but heavily armed blond hero (that cover appears to be an unused cover painting by artist George Gross, finished but never used for a Freeway Press reprint of the pulp magazine character Operator No. 5). In 2012, Moonstone Books finally published "Slaughter, Inc." in its original form as a Spider novel. "Slaughter, Inc." was also reprinted in its original form in 2018 by Altus Press in a facsimile edition designed to look like the unpublished issue #119 of "The Spider" pulp magazine. 
A year later, in 1980, Dimedia, Inc. reprinted three Spider pulp novels in the larger trade paperback format. In 1984, they continued with three mass market Spider novel reprints, one in 1984 and two in 1985. These last three sported new cover paintings of the original costumed Spider by artists Ken Kelly (on volumes one and two) and Frank Kelly Freas (on volume three). 
In the early 1990s, Carroll & Graf Publishers issued a series of eight mass market Spider paperbacks, each one in a double-novel format. All used original "The Spider" pulp magazine artwork for their covers. These 16 novels became the longest-running Spider reprint series done for the mass market paperback book market. One cover was a newly-painted version by Rafael DeSoto, the original artist.