Robert Silverberg 
Downward To The Earth 

One man must make a journey across a once colonised alien planet. Abandoned by man when it was discovered that the species there were actually sentient, the planet is now a place of mystery.A mystery that obsesses the lone traveller Gundersen and takes him on a long trek to attempt to share the religious rebirthing of the aliens. A journey that offers redemption from guilt and sin.This is one of Robert Silverberg’s most intense novels and draws heavily on Conrad’s Heart of Darkness. It puts the reader at the heart of the experience and forces them to ask what they would do in the circumstances.First published in 1970