'Heart of Darkness' is one of those 'story told within a story' stories. Frankly, an uncomfortable book to read on account of it's realistic first-hand accounts of the exploitation of Africa. Though a short book, it doesn't withhold blows describing 19th Century British attitudes to Africa, Africans and the environment; none of which are pretty.
The clever title can be applied to most issues raised in the book. Is the darkness the depths of the unknown, the attitude of those who would exploit others or does it dwell in seed form within the hearts of all men?