|Stolen mercilessly from Wikipedia -|
Merritt in about 1920.
Merritt is an interesting example of the way in which SFF has changed over the years, and how these changes have led to the bizarre invisibility of influential writers.
Merritt's bibliography isn't all that long compared to his contemporaries in "fantastic fiction", but he started early and was weighty enough that he was the subject of an interview in Science Fiction Digest's "Titans of Science Fiction Series" (Jan 1933) and the inimitable Forrest J Ackerman reported that when Merritt's day job prevented him from attending the 1st Worldcon in 1939 a group of fans visited him at The American Weekly offices afterwards.
Oddly, despite being credited with inspiring Lovecraft  and other contemporaries such as Richard Shaver, and Hannes Bok (not to mention several authors of the "Golden Age" to come), being listed on "best ever" lists for horror and fantasy/science fiction as late as the 80s , having been translated and reprinted in other languages decades after his death , being voted "favourite author of all time" by readers of Argosy in 1938, having his name in the title of a magazine  and creating an enduring cult masterpiece in The Snake Mother , Merritt's name rarely comes up these days. How could such an influential author be so generally unknown today?
Part of the answer probably comes from his writing - Merritt's rich prose is without question quite different from the rather spartan styles that came to be the norm in the 50s and 60s. Like many other writers from the first half of the 20th century, the density of Merritt's writing is such that for many people it's a love it or hate it proposition. Not that this rich style has entirely fallen out of fashion - not only have other authors of the period and with similarly "purple" style continued to be popular right up to today (um...Lovecraft?) but other writers continued to see success with richer language than the blunt "Hemingway-esque" style that began to dominate in the 50s and 60s (eg Jack Vance) and of course many well regarded modern writers work hard to achieve rich, lyrical prose. 
But even accounting for changes in stylistic tastes, by all rights, you'd think that Merritt would continue to enjoy some level of popularity - so why is it that according to my "rigorous" research  there have been next to no reprints of Merritt's work this century? (and few at the end of the last)
I think the answer probably lies at the feet of the same forces that resulted in "The Great Delisting" which I've mentioned before. 
"The Golden Age" of science fiction is typically thought of as being the the 1940s just after John W. Campbell took over editorial control of Astounding, but like Robert Silverberg  I prefer to think of the 50s as the true golden age of SF. In part, this is because some of my absolute favourite SF authors begin to emerge in this era, but mainly it's because this is when I start to see the cracks forming.
In the 1950s, Campbell's grip on the fate of science fiction was unshakable - he had total editorial control of Astounding, and Astounding was the premiere periodical of the time, with the best pay rates and the highest profile. It goes without saying that Campbell's vision of what science fiction should be was enormously influential. This is the era in which "technological visions of the future" increasingly became the very definition of science fiction, and this is when the heirs of Campbell's first generation of writers really came into their own.
At the same time, Tolkien's Lord of the Rings was making waves - and creating a template for "fantasy" as the fiction of magical pasts in contrast to the scientific futures being painted in "hard" science fiction. While fantasy elements continued to play a part in science fiction for some years  it seems as though the two sub-genres were increasingly in opposition, and creating alternate worlds that were increasingly isolated from one another.
The New Wave era of the late 60s and 70s made an effort to re-blur the lines between fantasy and science fiction, but really it didn't stick - by the time the RPG boom of the 1980s was underway (and in some ways because of it) the categories were becoming increasingly ossified, at least in terms of commercial considerations:
There were shelves for science fiction in the bookstores and the libraries, and on them you expected to find Niven and Heinlein and other tales of spaceships and ray guns and the future.
There were shelves for fantasy, where you might or might not find reflections of Tolkien's elves and hobbits, but you'd certainly find re-imagined pasts filled with magic and monsters, and faced with sword in hand.
There were shelves for horror - still often the horror of the occult and demonic forces, but also shading into the horror of blood and death, and certainly an increasingly distinct category.
This three-way split was by no means hard and fast - you still occasionally found crossover and blends in the 80s - but it was rapidly ossifying, in part due to the standardized business practices of the successful franchise chains that were becoming a powerful force in the publishing industry. 
Worse: fast forward to the 90s and later, and the genre categories get carved in stone by massive marketing efforts like Amazon where, with the challenge of browsing at all, it suddenly becomes critical for works to match the tags that will bring the right customers to them.
The thing about Merritt  is that while he was a highly influential writer of fantastic fiction, his work is rooted in a tradition that doesn't easily fit into modern genre categories. Most of his celebrated works are stories that might best be described as "fantastic adventure fiction" - the tales revolve around a contemporary (to the intended reader) protagonist who discovers something curious and follows a trail that leads to the discovery that "all is not as it seems." These are tales of the weird, of occult realities that lie hidden from ordinary people.
Which shelf do you even put this on in a modern bookstore?
While some of Merritt's contemporaries can be shoehorned into one of our modern categories or another - Lovecraft into horror, Ray Cummings into SF, for example - Merritt's development of this particular set of tropes is harder to sort. The closest we might see in the modern context might be something like Dan Brown or the X-Files, both of which deal with a similar kind of "contemporary protagonist discovers a hidden dimension of reality" kind of adventure story, but I suspect the style needed to carry off a modern "conspiracy theory" adventure piece is a discouragingly large step from Merritt's work.
Unfortunately, this leaves him without any clear heirs to claim his legacy - and more importantly, it leaves booksellers with a head-scratching problem: if they did reprint some of Merritt's work, where should it be displayed to ensure the right readers see it? What kind of cover art should it have to signal what kind of story it is?
And these considerations are above and beyond the basic question of whether the ordinary social norms reflected in Merritt's work will be acceptable to a modern audience. 
It's a shame that a combination of shifts in literary taste, narrowed definitions of genre, and certainly to some extent shifts in social attitudes  would render such an important writer nearly invisible to today's fans. Merritt is an author who definitely warrants a look by anyone who is interested in seeing where some of the common tropes of modern fantasy and horror come from - and luckily, although most of his work languishes in the unreprinted void of the Great Delisting a fair amount of it is out of copyright in some parts of the world and available online in one format or another.
Take a look, embrace the heavier prose of yesteryear - once you get used to it, you're in for a wild ride!
1. The Moon Pool and The Conquest of the Moon Pool which appeared in installments in All-Story Weekly 1918-1919 seem to have been the proximate cause of Lovecraft's Deep Ones (in The Shadow over Innsmouth (1931)), and some people see a strong link between Merritt's "The People of the Pit" (Jan 1918) and HPL's At the Mountains of Madness (also 1931)) - but it should also be noted that Lovecraft had harsh words for the way Merritt's style developed over the years:
"Abe Merritt - who could have been a Machen or Blackwood or Dunsany or de la Mare or M.R. James.... if he had but chosen - is so badly sunk that he's lost the critical faculty to realise it... Every magazine trick & mannerism must be rigidly unlearned & banished even from one's subconsciousness before one can write seriously for educated mental adults. That's why Merritt lost - je learned the trained-dog tricks too well & now he can't think & feel fictionally except in terms of the meaningless & artificial cliches of 2-cent-a-word romance. Machen & Dunsany & James would not learn the tricks - & they have a record of achievement beside which a whole library-full of cheap Ships of Ishtar & Creep, Shadows remains essentially negligible" -- From Lovecraft's letters: SLV.400-01 (7 Feb(?) 1937)
2. Moorcock and Cawthorn cite Ship of Ishtar as one of Merritt's best works in Fantasy: The 100 Best Books - curiously, quite at odds with Lovecraft's assessment of the work as representing the corruption of the periodicals (see the quote above) - and in 1985 a major collection of Merritt's work was published, including poems, essays and writings about him by others.
3. As just one example, Hayakawa Bunko published Seiji Dan's translation of The Snake Mother as one of their "classic SF" collection in 1971: http://kicchan.s19.xrea.com/img/hayakawasf.html cat.# SF0020 - sadly Hayakawa doesn't seem to still have this item on catalog - however a translation of People of the Pit by Jitsuko Haneda was published by Asahi Sonoda in 1986.
4. Seriously - the guy was popular: http://www.gwthomas.org/amerritt2.htm
5. Enduring enough that despite The Face in the Abyss being published in 1931, Snake Mother herself was the subject of winning costumes at least twice long afterwards - Myrtle Jones won the Pacificon masquerade with her Snake Mother costume in 1946, and Kathy Sanders appeared in the same guise - but with hand maidens! - at Westercon in 1980.
6. The difference of course is that many of these modern writers are aiming for something rather more like prose poetry - yes, the language is rich and poetic, but it sometimes seems almost as though "plot" has fallen out of fashion.
7. A scan of Amazon - which was subsequently contradicted by the discovery of a Merritt omnibus available on Google Play.
8. But have yet to expand on as promised - I'm working on it, I swear.
9. In his essay "Science Fiction in the Fifties: The Real Golden Age"
10. Even Campbell had a thing for telepathy and the like for a while.
11. Interestingly, it occurs to me that the big box "we have everything" bookstores like Chapters and Barnes & Noble were late in expanding to the UK - though it's obviously a franchise chain, I don't recall W.H. Smith having the same kind of approach at all - and perhaps this explains the fact that British imprints seem to have adopted firm divisions a bit later, if my admittedly porous memory of the UK in the 80s can be relied on.
12. And Lovecraft as well, actually - his popularity waned for a while before resurfacing in the horror boom of the 70s.
13. As an example, when Tor's "Advanced Readings in Dungeons and Dragons" covered Merritt, they raised some concern about treatment of race and women - to be honest, my sense is that while attitudes of the time were of course quite different from today, the truth is that Merritt's treatment of women and minorities was really quite liberal for the period. In addition to making allowances for social changes over the more than a century since his birth it's probably also important to take into account the fact that he was almost constantly riffing on the "lost civilization" trope standardized by Haggard - as such it's expected that we will have an intrepid explorer (probably a "colonist") who then encounters a variety of "aboriginal" people. In Dwellers in the Mirage, while a bit patronizing the depiction of First Nations people is quite positive, while the story itself revolves around proto-Aryan invaders who worship a demonic god - it would be simplistic to dismiss Merritt's work as racist.
14. Honestly, I don't think political considerations like worrying about unacceptable attitudes toward race, gender, class etc