Merritt's obscurity to contemporary readers may be because he produced comparatively few works of fiction. By profession, he was a journalist and editor rather than an author, working for most of his career for The American Weekly. Indeed, he was one of the most successful -- and highly paid -- journalists of his era, making, according to Sam Moskowitz, $25,000 a year in 1919 and $100,000 a year at the time of his death in 1943. It's little wonder, then, that Merritt wrote far fewer stories than, say, Lovecraft or Howard. That he nevertheless did so testifies, I think, to Merritt's passion for the fantastic and the weird, a passion that shines through in his best stories. He used his wealth to travel the world, seeking out the strange (he was an early member of the original Fortean Society) and he assembled a large library of the occult.
H.P. Lovecraft was a great admirer of Merritt and even had occasion to meet him in person in 1934, while he was visiting New York, where he maintained a home on Long Island. He describes his impressions of the man in a contemporary letter to Robert H. Barlow:
I really had a delightful time — seeing all the old gang… But the meeting which will interest you most is that with A. Merritt, the Moon Pool man. It seems he had long known my work and held a very kindly opinion of it. Hearing of my presence in NY he took steps to get in touch with me, and finally invited me to dinner at his club — the Players, which occupies Edwin Booth’s old home in Gramercy Park. Merritt is a stout, sandy, grey-eyed man of about 45 or 50 — extremely pleasant and genial, and a brilliant and well-informed conversationalist on all subjects. He is associate editor of Hearst’s American Weekly, but all his main interests centre in his weird writing. He agrees with me that the original Moon Pool novelette in the All-Story is his best work. Just now he is doing a sequel to Burn, Witch, Burn (which I haven’t read, but which he says he’ll send me), whose locale will be the fabulous sunken city of Ys, off the coast of Brittany. It will bring in the comparatively little-known legendry of shadow-magic. Merritt has a wide acquaintance among mystical enthusiasts, and is a close friend of old Nicholas Roerich, the Russian painter whose weird Thibetan landscapes I have so long admired. I was extremely glad to meet Merritt in person, for I have admired his work for 15 years. He has certain defects — caused by catering to a popular audience — but for all that he is the most poignant and distinctive fantaisiste now contributing to the pulps. As I mentioned some time ago — when you lent me the Mirage installment — he has a peculiar power of working up an atmosphere and investing a region with an aura of unholy dread.I like this passage in HPL's letter not just because of what it says about Merritt as a writer but as a man. By all accounts, Merritt was a pleasant, well-read, and unpretentious person who did not allow his success to distance him from the things he loved. One of America's highest-paid journalists, who used his influence to, for example, promote the work of then-controversial physical therapy pioneer Elizabeth Kenny (which, coincidentally, greatly aided the creator of the Illuminatus! trilogy, Robert Anton Wilson, as a child), Merritt neither felt shame nor saw any contradiction in his weird fictional side career -- an attitude I find laudable and wish more gamers might adopt as their own.
There are many common themes in Merritt's fiction, most notably lost races and subterranean realms. Such ideas are commonplace now, in part because of the role Merritt played in promoting them, both through his fiction and his editorial work on The American Weekly, which often included stories of scientific "marvels" and inexplicable events. Lovecraft readily acknowledged the debt he owed to Merritt's stories, particularly The Moon Pool (which, interestingly, was also noted by Gygax as a favorite of his), so there's a sense in which the Mythos as it came to be might not have existed without Merritt. Similarly, Richard Shaver, most famous for having written "The Shaver Mystery" in 1947, seems to have believed that Merritt's stories were not in fact fiction but rather fictionalized accounts of things Merritt had actually experienced while on his world travels. Shaver's own theories of subterranean realms and ancient races were themselves widely influential, including on Dungeons & Dragons, whose derro race is certainly derived from Shaver's own "dero" (short for "detrimental robot," called such because of their robot-like behavior).
In my opinion, both Merritt and his fiction deserve much wider recognition than either has received in recent years. The more I learn about him and his life, the more fascinated I've become and his fiction, while not of universally high quality, nevertheless contains enough excellent entries to justify spending the time to read them. That he was claimed as an influence by many of fantasy and science fiction's luminaries (not to mention Gary Gygax) is another reason to give him a look, if you've never done so. I don't think you'll regret it.