Some books are classic because they are great. Some books are classic because they were the first to do something, and it doesn't matter how well it was done. Then there are the books that are classic for no discernible reason. Moon Pool belongs to this category. It's a Journey to the Center of the Earth-type story, so it certainly wasn't the first of its kind. And it isn't great by any measure. Yet it is mentioned often in works describing the early history of science fiction as a genre, so dutifully I read it. Let me save you the trouble: skip this one, and go back to reading Edgar Rice Burroughs stories if you want to read sf/f from the 1910s.
The Moon Pool, despite its name, does not take place on the Moon, but instead occurs mostly in a lost world inside the Earth. It is interesting to compare the siting of this lost world to those of H. Rider Haggard. He put his lost realms in remote corners of Africa, satisfied that the readers could suspend their disbelief that such fantastic realms had not been previously discovered by Westerners. With the advent of the airplane, however, authors had to get more creative with their new frontiers. After all King Solomon's Mines wouldn't have any suspense at all if Quatermain's band had simply flown in, seen the situation, and flown back to put together an air raid. So when Merritt looks for a place for his lost world, he has to put it somewhere that airplanes can't go, i.e. underground. Considering that E. R. Burroughs had already done this with At the Earth's Core (1914) and even better, gone to Mars with John Carter in 1912, Moon Pool in 1919 seems rather behind the times.
The plot is that a scientist traveling around Indonesia hears an alarming story from an obviously distressed friend, who then disappears. Investigating the story, he finds himself on an island with a slightly lunatic American aviator, a distraught Norse ship captain, and a sinister Russian scientist. They unlock a hidden cavern, and are transported into a deep subterranean realm. There they get involved in the local politics, run afoul of the local gods, fall in love with the local women, etc. The narration is all from the first person perspective of the expedition's only returnee, the scientist Dr. Goodwin. His style is as dry and annoying as any stereotypical scientist one may care to imagine. Merritt takes great pains to be true to his narrator's voice. We see that he can write better, since the dialog from the loony Irish-American pilot is full of life and energy. Unfortunately, Dr. Goodwin is the least interesting and readable possible narrator, and his voice makes every page a slog. Here's an example of his excited narration:
And the Shining One drew back!
Yes, drew back--and back with it stepped Yolara, the doubt in her eyes deepening. Onward paced the handmaiden and the O'Keefe--and step by step, as they advanced, the Dweller withdrew; its bell notes chiming cut, puzzled questioning--half fearful!
Even though much of the story is told in dialog, even most of the dialog (that isn't O'Keefe's) is tendentious. Here's an infodump portion from the main love interest:
"In the Shining One had grown craft, cunning; knowledge to gain that which it desired. Therefore it told its Taithu--and mayhap it told them truth--that not yet was it time for them to go forth; that slowly must they pass into that outer world, for they had sprung from heart of earth and even it lacked power to swirl unaided into and through the above. Then it counselled them, instructing them what to do. They hollowed the chamber wherein first I saw you, cutting their way to it that path down which from it you sped.
This goes on for page after page. Science fiction is known for awkward infodumps, but at least when Yoda's explicating things with weird phrasing, he's also quick about it.
There are some interesting points here amongst the tedium. For one, Merritt is more aware of world cultures and mythical references than is common even today, leading to occasional passages such as:
Dimly there crept into my mind memory of the Dyak legend of the winged messenger of Buddha--the Akia bird whose feathers are woven of the moon rays, whose heart is a living opal, whose wings in flight echo the crystal clear music of the white stars--but whose beak is of frozen flame and shreds the souls of unbelievers.
Also, there is a spiffy way of eliding the requisite "scientific" explanations of obviously fantastic things: when Dr. Goodwin appears to be starting to launch into an actual scientific explanation, the editors of the fictional scientific committee publishing the narrative step in and explain that they are removing the explanations lest they fall into the hands of the Russians.
Despite moments of in interest, this book is too long and too tedious to bother with. There are annoying plots indicating that women having too much power is a bad thing, just like H. Rider Haggard's She, which I also disliked. There are lots of religious themes, and it starts to follow a Christian good vs. evil path, but it doesn't resolve in a satisfying way. In the end the author simply cheats on his foreshadowing, frequently mentioning things like "the last time I saw him alive...," "I never saw them alive again..." and then not following through. The ending, explaining how Dr. Goodwin returns to the surface to present this testimony, is cheesy in the extreme. It's only one degree better than "and then Timmy woke up."
The best thing about Moon Pool is that its copyright has expired and it is thus available for free from Project Gutenberg. This means that if some masochistic impluse causes you to read it, at least you won't waste money on it. Alexei Panshin, in his study of the history of early sf/f The World Beyond the Hill is quite taken with Merritt's works, noting especially how he blends mysticism and scientific impulses in his stories. That may be, but before one gets to that level a book should be readable and enjoyable on some level, and Moon Pool fails on both counts.