David Lindsay published this in 1920, and Ballantine re-issued it in the ’60s when they and everyone were waiting for Tolkien to finish the Silmarillion. I just re-read it; where Eddison is hit-or-miss, Lindsay is remarkable. Better than the Silmarillion, in fact.
I made the map below for my own use when I read it as a teenager. I wish I had a ball-point pen that fine today.

Lindsay has had many admirers, of many types: Alan Moore, Michael Moorcock, Harold Bloom, C.S. Lewis. Lewis notes that a reader may appreciate the weirdness shown in the first chapter, but will expect that it can’t be sustained:
Tormance, when we reach it, he forbodes, will be less interesting than Tormance seen from the Earth. But never will he have been more mistaken. Unaided by any special skill or even any sound taste in language, the author leads us up a stair of unpredictables. In each chapter we think we have found his final position; each time we are utterly mistaken. He builds whole worlds of imagery and passion, any one of which would have served another writer for a whole book, only to pull each of them to pieces and pour scorn on it. The physical dangers, which are plentiful, here count for nothing: it is we ourselves and the author who walk through a world of spiritual dangers which makes them seem trivial.
Lewis is quite right. But let me step back a moment and set up the plot. Some posh Londoners are having a séance— the psychic, Mr. Backhouse, is a dour, uncharismatic man who promises a spectacle and duly produces one: a man materializes in the room. But an uninvited guest, Krag, mocks the apparition and snaps its neck. He then goes up to one of the other guests, Maskull, and asks him, “Wouldn’t you like to see the land where this sort of fruit grows wild?”
Maskull is interested enough to follow the man outside, along with his friend Nightspore. (Yes, Lindsay’s names are rather gothic.) Krag explains that he and Nightspore are traveling to Arcturus— specifically to its planet Tormance.
Well, it wouldn’t be much of a story if Maskull declined. He and Nightspore travel to a Scottish observatory where the trip is to start. Putting aside some atmospheric intrigue (the place seems long deserted): Krag appears with bullet-shaped vessel. They will use “Arcturan back-rays” to travel. These are light rays that strain to return to their source; they are used to propel the vessel. Maskull falls asleep on the journey.
He wakes up naked and alone in a red desert, with scattered purple plants. Tormance is larger than Earth, so the gravity is overwhelming. Fortunately a native rescues him— a woman named Joiwind. By sharing blood, he is enabled to stand up and walk.
Tormance is one of the strangest planets in sf. Lindsay is no scientist, but he has plenty of striking ideas. The inhabitants are humanoid, but they— and Maskull— develop special organs: extra eyes, tentacles. One belongs to a third sex, the phaens, and gets new pronouns (ae, aer). There’s water you can walk on, terrain subject to brutal rearrangements, horse-sized insects, wheeling three-legged animals, trees that trap large animals. There are two new colors: jale and ulfire. You see, there are two suns; one has red, yellow, and blue as its primary colors, and the other has blue, jale, and ulfire. (As it happens, “primary colors” happen in our eyes, not in light, but it’s actually correct that if you had the receptors to see extra colors, you’d get them in pairs.)
(If you’ve read Out of the Silent Planet, Lewis’s sensoriness owes something to Lindsay. Most sf authors want to get down to colonizing or shooting or whatever, and neglect to tell us what it’s like to be on a completely alien planet.)
The structure of the book is episodic: Maskull meets one or two natives, talks with them, often develops whatever local organs they have. (Some of these special organs account for everyone’s ability to speak to each other.) More importantly, each of the natives expounds an entire way of life, generally completely contradicting whatever the last one believed.
Joiwind, for instance, is a loving and benevolent soul— she has sensed Maskull’s arrival and walked many miles to rescue him. She lives on nothing but water, believing that even to strip a leaf off a plant is criminal. On the other hand, he soon meets Oceaxe, who comes from a land where the ego is king, and people do only what will benefit them.
If this is beginning to sound like a morality story— no, it’s much deeper and more surprising than that. Lindsay is very unusual in being able to portray characters of wildly differing moralities and let them explain themselves as well as they can. The reader can judge them, but none of them is the sort of cardboard figure that most authors produce when depicting someone of an ideology they dislike. Generally Maskull learns to act in accordance with the local mores; as he is a new element, this often leads to change and tragedy. He keeps moving, looking for answers. Here there’s a society of men only that despises all pleasure; here’s an musician whose art is so powerful that it kills anyone who listens to it; here’s a phaen who is driven to find a spiritual underground world though it is sure to kill aer…
All of this works surprisingly well: Lindsay doesn’t run out of novel philosophies, nor odd characters to explain them, and Maskull is a perfect foil— each encounter changes him, not always for the better, and compels him to act.
The show has to end somewhere, and one might presume that the final chapter contains Lindsay’s final point of view. What’s presented is a sort of Gnosticism— the universe is described as created by one god, but corrupted by another. And maybe that’s exactly what Lindsay believed— but I doubt it; books meant to end with a particular ideology of any kind usually get there much faster, and treat the alternatives far less graciously. Though Lewis talks about him “pouring scorn” on each viewpoint as he leaves it, that’s not the feeling I get. I don’t think Lindsay is writing a Gnostic tract with instructive moral tales about the failures of non-Gnostic approaches. It’s more like a catalog: look, here’s how the self-sacrificing and compassionate Joiwind speaks and acts; here’s how the entirely self-serving Oceaxe thinks and lives.
I didn’t find the last chapter satisfying, but I’m not sure what would have worked instead. Tales of spiritual journeys are interesting only until the point when the protagonist has all the answers: even if you accept the final destination, the genre is about the doubts and slips along the way.
As well, up to that point the book succeeds despite almost entirely ignoring the normal notions of plot and character. Maskull wants to find out all he can about Tormance and its God or gods. That sets up the catalog, but it’s not a plot, and a childish part of me rebels at the end because, as a plot, the end makes no sense. Oh well… I usually find the endings of video games unsatisfying too.
Edit: There’s one near-constant in the ideologies Lindsay catalogs: suffering and sacrifice. That’s the one bedrock value that he seems to have, and it’s why so many of the stories end in death: the only worldviews worth having, to him, are the ones you’d die for. Almost the only exception is the land of egotists (including Oceaxe), and it’s hardly necessary to add any moral condemnation there; he simply shows the natural consequences of their views.
I should warn modern readers of one thing— Lindsay likes to play with gender essentialism. This doesn’t mean that he’s misogynistic (though some of his characters are). If anything, he does well with his female characters— and this is a rare classic sf novel which has plenty of them. And it’s worth remembering that feminism, circa the seventies, used to dip heavily into gender essentialism itself. But it’s in disrepute today, for good reasons— no one should be limited by what someone else thinks their sex should be like.
Since I quoted Lewis’s disparaging remarks on Lindsay’s style, I should also add that I don’t agree. I find Eddison’s style disagreeable; Lindsay is straightforward and quite readable. The wonder is in the ideas; there’s no need for him to dress up the style as well.
It’d be interesting to make the book into a video game. Faithfully, I mean— at least, as faithfully as it could be done without a monitor that can properly render jale and ulfire. It’d mostly be a walking (and talking) simulator. Imagine having to walk across the desert with Joiwind, for maybe an hour, and if you stop too many time to talk to her you both weaken and die. I think it’d be a big big hit.