Downward to the Earth

This is the fourth of Robert Silverberg's novels that I have read to date, all of which were published from 1969 to 1972. He is becoming a favorite author. I've read reviews of Downward to the Earth comparing it to Heart of Darkness. I see that to a certain extent, though I like Silverberg's writing better than Joseph Conrad's. I can also imagine its similarities to an unwritten sequel to Ursula K Le Guin's The Word for World is Forest. It is certainly similar to Silverberg's own The Book of Skulls in that the protagonist quests to undergo an unknown and potentially horrific spiritual metamorphosis.