A science fiction first published in 1969, written by the then-immortal Robert Silverberg.
The title is from Ecclesiastes 3:21:
Who knoweth the spirit of man that goeth upward, and the spirit of the beast that goeth downward to the earth?
And that's what it's about: The spirts of men and beasts and where they end up. The question which is asked, and partially answered, is the obvious one: "Which is which?"
This thing is relatively tame by the standards of the SF "New Wave" of the 1960s (as opposed to all the other new waves which have beset film, music, and God knows what else), but Silverberg was by that time an oldster. He started writing in the 1950s. When the field began blooming in all strange directions, he took what suited him from the younger writers and ignored the rest. He never indulged in the kind of excesses that doomed Norman Spinrad and Robert Anton Wilson to an eternity as one-trick novelty acts, the drug-fiend Henny Youngmans of science fiction.
Downward to the Earth is very (very) loosely based on Heart of Darkness. It's not a retelling of the story in a different context. Rather, Conrad's novel is evoked as contrast and commentary and -- dare I say it? -- "subtext" (or something). I can't think of anything meaningful to say about it all in terms of Apocalypse Now (1976?), but I guess that one has to be mentioned.
Where Conrad's novel was colonial, Silverberg's is post-colonial. He was writing in the late 1960s, remember. A jungle planet was colonized and commercially exploited, and then it was peacefully returned to the natives (much like the British Empire gave up so many possessions around the world when European imperialism went out of fashion). A man named Edmund Gunderson, a former colonial administrator for the Company which ran the planet, returns to lay some ghosts to rest. A few humans remain on the planet because they can't bear to leave, and there is a small tourist industry. In the old days, there was a Mr. Kurtz who went out to the bush to trade with the "natives", and there was great corruption. We depart from Conrad in that the corruption is more explicitly an imported corruption, inflicted on the natives. It's Kurtz, not them, who always had uncontrollable and "primitive" destructive urges. The natives, being aliens (there are two varieties, one being three-meter-tall hirsute humanoids, and the other resembling elephants), appear more bestial than Conrad's Africans, but Silverberg gives his natives more credit for being real people than Conrad often gave his own. Again, this is the late 1960s and attitudes had changed.
Gunderson had known Kurtz when Kurtz was a brilliant young hotshot, and knew him as things began to turn strange. On returning, he sets off into the brush to find Kurtz, who is known to have remained behind. He re-lives old memories, many of them strange and terrible, with Kurtz figuring in some of the strangest and most terrible. He meets a former lover who lives alone at a waterfall; she has changed and they can no longer really speak to each other. He finds the ruins of Kurtz's old station, deep in the forest. He meets natives, some of whom he had personally and violently wronged in the old days. He hears that Kurtz has vanished into the mysterious northern hemisphere, intending to undergo a native ceremony called "Rebirth". None of the humans have ever known just what that word means to the natives, and none of the natives are talking.
Gunderson follows Kurtz. In the end, we learn that "Rebirth" is considerably more than just a ceremony. It's a physical transformation governed in part by the mind of the subject. Kurtz was a moral cripple and it changed him horribly. He lies dying on a cot in a mist-shrouded alien village, still having no real understanding of himself or anything else. It becomes clear that the humans never even began to understand the planet they thought they owned. Kurtz is beyond redemption, but Gunderson makes peace with himself, with his past, and with the natives. It's not exactly a happy ending, but it's just.
It's a rare science fiction writer who can put that much meaning and morals into a novel, and still write an interesting narrative with compelling and real characters, both human and alien. This is a damn good book.