Ondaatje is likely referring to Joseph Conrad, born in 1857, the author of the famous novella Heart of Darkness. The story follows Marlow, an Englishman, on a foreign assignment as a ferry-boat captain in Africa. While employed to transport ivory downriver, Marlow is also assigned to return Kurtz, another trader, who has established himself as a god among the natives.
You can read Heart of Darkness, originally published in three parts, here.
For further information, take a look at the Wikipedia page.
The most famous adaptation of Heart of Darkness is Francis Ford Coppola's film, Apocalypse Now, which moves the setting of the story to the Vietnam War.