Abraham Merritt is one of those authors who's more talked about than read nowadays, assuming one even knows his name at all, which isn't likely. That's unfortunate, because, in his day, Merritt was incredibly influential, with H.P. Lovecraft -- and Gary Gygax -- among those who took inspiration from his writings. Indeed, Lovecraft notes on several occasions that he had specifically saved the June 1918 issue of The Argosy magazine that contained a fantasy novel by Merritt entitled The Moon Pool.
If you've had the chance to read The Moon Pool, you'll quickly see why HPL was so fond of it. The novel tells the story of a scientist (Dr. Throckmartin) who encounters an old friend (Dr. Goodwin) on a ship while they are both sailing back to New York. Throckmartin tells Goodwin of his recent, disastrous expedition to ruins on the island of Ponape -- a name familiar to Lovecraft fans -- in which all of his companions, including his wife, were abducted by a mysterious being of light he has dubbed the Dweller. Throckmartin claims that this being continues to pursue him and, sure enough, when moonlight falls upon him, he is whisked bodily away, right before Goodwin's eyes, who vows to rescue his old friend from whatever peril has overtaken him. Goodwin assembles a multi-national team of scientist and adventurers to accompany him on his quest and soon finds himself exploring a lost subterranean city that is home to not one but two strange races with equally strange religious practices. Needless to say, Goodwin and his companions discover that not only the fates of Throckmartin and his expedition hang in the balance but that of the entire world.
The Moon Pool is, on the face of it, just another "lost world" fantasy, a theme to which Merritt regularly returns in his writings. Being Merritt's first novel, it's also a lot less polished than his later works. Yet, it somehow manages to overcome its structural and stylistic weaknesses and command my attention. Part of it might be the way that the story takes lost world tropes a step further by introducing a level of "cosmic horror" that you don't find in say, Arthur Conan Doyle. This is clearly what Lovecraft saw in the story. There's also the fact that The Moon Pool includes if not the first, then one of the earliest examples of the classic pulp adventuring team made up of a rogues gallery of guys with different backgrounds and skills united in a common cause to save the world. One could even stretch this a bit and see the origins of the D&D adventuring party.
In any case, The Moon Pool is a good introduction to Merritt if you've never encountered his work before. It's not my favorite of his books, but it's still excellent. It's also one of only three Merritt books which Gygax cites by title in the Dungeon Masters Guide, so it's certainly worth the time of anyone interested in the literary origins of the game.