If I remember correctly, it's about a news reporter who is traveling down the Congo river to meet up with a manager (or someone of the sorts) who has been really successful in mining operations upriver. Turns out the guy has gone a bit insane and has made up his own little empire.
Someone please let me know when I am mixing up the plot of the book with that of the movie inspired by the book (I'm not gonna tell and facilitate someone just watching the movie, and no it's not the same title as the book)