Early science-fiction? You may not agree, but there’s an automaton that drives the protagonist insane (more or less, shush!), so I’m declaring it sci-fi, on the horror side of things.
In itself, the book sets the Enlightenment and Romantic movements against each other, much like Frankenstein (published a couple of years later). Additionally, it is an early example of psychological horror stories, and experiments with disjointed narratives to mirror the terror (or insanity, depending on your interpretation) of the protagonist.