The University of Texas has a number of intersting essays about a variey of topics realating to Conrad's novella. Look for "Light vs. Dark," "Racism," and "Superiority."
Hypograms, Hypocrites, and Hippos
The symbol of the wilderness in Heart of Darkness
How To Cook a White Man. Probably not really relevant to serious research, but should be included just for its title. Who knows, there just might be a Kurtz recipe book in there.