If you're interested in symbolist literature, Against Nature is a must read. Otherwise, there's little reason to bother. The plot is virtually non-existent and what plot exists is all interior. It concerns a wealthy man named Jean Des Esseintes, who has been pampered all his life and is a bit of a fop (the technical term of the time was 'dandy'). He and his friends roam Paris indulging their sexual and sensual whims. This passage of his life ends with him literally burning out on life in the fast lane, and he becomes increasingly disgusted with society. He decides to retreat into a house where he can seal himself away from all human contact, sleep all day, and wake at night to read his books, revel in his art, and commune with his thoughts.
Interesting? What I described is just a small section of the novel. The bulk of it is given over to largely descriptive chapters that go into extreme detail over Des Esseintes meticulous efforts to create and enjoy his private world of illusion. Imagine the first few pages of an Edgar Allan Poe short story such as "The Fall of the House of Usher" or "The Telltale Heart" in which we are acquainted with the perverse, neurosis of the narrator and now stretch that into the course of a novel, delving into the particulars of this kind of character. Against Nature can be very slow going; too slow to be recommended as a casual read (at just under 200 pages, it took me several weeks to get through it).
Count Robert de Montesquiou (1897)
And my reaction is not far off. The critical assessment of Against Nature refers to it as a bible for the Decadent movement. There are entire chapters reviewing color in decor, literature, music, art, philosophy, and even flowers in terms of how they fit with the isolated, decadent life Des Esseintes builds for himself. This is punctuated by brief vignettes illustrating his eccentric dandyism. He buys a tortoise as a sort of pet, and then has its shell gilt with encrusted jewels he picks by hand to achieve the right aesthetic impact. Elsewhere, he meets a young hustler and exposes him to the pleasures of brothels as an experiment in "producing a murderer".
It's not long before the isolation affects Des Esseintes mentally and physically. He becomes increasingly neurotic and lethargic. It is the description of his mental decline that most loudly echoes the works of Poe, and in fact Huysmans has Des Esseintes repeatedly reference Poe by name throughout the text as a prime example of what he is after. Towards the end of the book when doctors are summoned to save him, Des Esseintes describes his mindset succinctly:
"The doctors spoke of amusements and distractions; but with whom, and with what, could they possibly suppose that he might amuse or enjoy himself? Had he not outlawed himself from society? Did he know one man capable of trying to lead a life such as his own, a life entirely confined to contemplation and dreams? Did he know one man capable of appreciating the delicacy of a phrase, the subtlety of a painting, the quintessence of an idea, one man whose soul was sufficiently finely crafted to understand Mallarme and to love Verlaine?"
This brings us to the theme that may exist in this pile of description and obsessive fantasy. The core idea is about the man of depth and intelligence who sees the world around him slowly rotting intellectually. This decay is brought about by the standard 19th Century demons: the rise of a middle class who are crass and vulgar, who buy art for show not because they understand it, who attend plays for base entertainment without expending thought as to the meaning of what they experience, etc. He describes it as leading to "the crushing of all intelligence, the negation of probity, the death of all art." He also blames the rise of capitalism and it's incessant focus on money and acquisition ("the vast whorehouse of America, transported on to our continent").
Huysmans wrote an Appendix to Against Nature twenty years after the novel was published. By this time, he had rejected his decadent ways and become a full blow Catholic. He essentially apologizes for his novel and suggests that the theme he surfaced in Against Nature can only be solved through a choice "between the muzzle of a pistol and the foot of the cross". Suicide or mysticism. He chose mysticism, claiming religion gives man hope. However, the alienated state of mind he - and other writers of the time - presented is the precursor for the rise of modern literature in the 20th Century in the form of Camus, Sartre, Kafka, etc. They chose neither option and, instead of clinging to what they would consider a cotton-candy kind of hope offered by religion, advocated embracing the absurdity of life directly.
For me, this is the most interesting aspect of Against Nature, Bruges-la-Morte, and other novels of this period. It's the preliminary thinking that would lead to the explosion of revolutionary creativity in the first half of the 20th Century.