This is a complex book, which can be read in many ways. That said, however, it just didn't resonate with me. It either attempts to do too much, or too little, depending on your perspective. Its value lies in what it overtly reveals, rather than in the surface story, which is rather superficial, so it's a book that should be read carefully, and, perhaps, repeatedly. On its surface, it tells the story of Marlow, a man with few prospects, who journeys to a faraway land to become a riverboat captain in what's generally accepted to be the Belgian Congo in search of a fellow ivory trader who has "gone native." The novel is certainly a product of its time, having been written in the late nineteenth century, when the British Empire was in full-swing. It first appeared not as a novel, but as a serial in "Blackwood's Magazine" to celebrate its thousandth edition.
Many people became familiar with at least the overall notion of the book via the immortal classic film "Apocalypse Now," which was at least inspired by the book, but I would only use the term "inspired," as there are so many differences, both in form and substance, that the film is certainly its own masterpiece. Both stories occur during times of upheaval in the wake of colonialism, Vietnam and the Belgian Congo, but the setting's quite different. The novel also has some "Moby Dick-esque" elements to it, specifically the obsession Marlow has with finding the ivory trader, Kurtz, and the psychological devolution which follows his relentless pursuit. The notion of a young man, drawn to the "blank spaces" on the map, as Ishamel, who desires to go to sea for adventure and fortune, also strikes a chord.
To me, the book just lacked substance: notwithstanding its veiled messages, the prose just wasn't engaging. Even the introduction in this edition is insightful, but interminable. Some have even described the book as "Modernist," specifically on account of its "post-structuralist" approach to notions of language, i.e., Conrad's frequent use of terms such as "unspeakable," "indescribable," which convey the message that language fails to describe the horror the character is attempting to convey. English was Conrad's third language (he was Polish, and spoke fluent French), but he was still quite fluent in English as well, so there is at least some merit to this claim. I'm still somewhat dubious of this overall claim, however. I think that heralding this work of the late-nineteenth century as "post-structuralist" is overreach, and is definitely over-reading the text, but I have to admit that I'm not certain how to classify it.
Back to interpretation: many have rightly heralded the book as a critique of European colonialism before such criticism was generally accepted, hence the rather veiled nature of the overall message. Conversely, it was famously critiqued by Chinua Achebe, a Nigerian author, in the 1970s, who attempted to view it through the lens of postcolonial analysis. It's become a popular read in postcolonial studies for that reason, which was where I first encountered it, specifically because it IS overt in its message of how the colonizers viewed and treated native peoples, as Achebe noted. Scenes of mistreatment, specifically torture (i.e., a man accused of having started a fire which destroyed the Company's goods) are probably an accurate representation of the situation on the ground at the time, as they still are today in many post-colonial areas of conflict.
As such, the novel provides a convenient place from which to explore colonialism: it could also be seen as an exploration of notions of superiority and inferiority, even in the mind of the author. For example, the character Marlow seems to divide everyone into two main groups: Europeans and Africans, civilized vs. uncivilized, the latter who appear as dark, exotic, untrustworthy (Marlow even dismisses many of his own crew as "cannibals"). The effects on Marlow's psychological well-being become increasingly obvious the further he travels into the unknown, in search of someone who ostensibly suffered the same fate.
Others have lambasted it for its rampant and sometimes-shocking racism, eroticism and "fetishization" of native peoples, and exoticism ala Said's "Orientalism," focusing specifically on the "dark" continent and the people who live there. That's somewhat alluded to in the title, but it's more complex than that. Many have also noted its rather overt misogyny, which was also a prevailing attitude at the time, specifically Marlow's dismissive attitude of his aunt, and Kurt'z partner, whom he considers to be a sheltered woman with no idea of what goes on in the world around her. The story, perhaps not unexpectedly so, is almost entirely devoid of women, told from the male perspective for other males, in a world where women are essentially invisible.
It's unclear to what degree the novel describes the author, Conrad (although several of his others are likewise revealing). Perhaps the author, who for a time also served as a merchant marine, experienced some of the trauma that Marlow describes, and, as such, describes his own heart of darkness.
"Droll things life is- that mysterious arrangement of merciless logic for a futile purpose."