Heart of Darkness by Joseph Conrad
"Heart of Darkness" follows an Englishman, Marlowe, on his travels into the Congo. He has been hired to ferry ivory from upriver, and bring back an ill station manager, Kurtz. He becomes fascinated with Kurtz' reputation. Kurtz is seen as a highly able man who is extremely successful at supplying ivory.
Marlowe is forced to spend several weeks at the port awaiting supplies to repair his boat. He's horrified to see the way the natives are treated. They're disposable. Barely fed, and badlly mistreated. Disposable. They're considered subhuman.
Marlowe is horrified with everything around him and events on the trip are no better. But when he finally meets Kurtz, he truly sees the heart of darkness.