An enjoyable novel. I am a sucker for spatiality and questions of gaze, eyesight, in literature (too much Dostoevsky / Gogol), so off the bat I knew this would be ripe for my pilfering in some way. This is likely rambling but I've edited too much as it is. No outright spoilers but I talk of the broad themes, which might count as spoilers depending on how you see things.

As to how it stands in comparison to other French Decadents, I would be unable to say for sure since I am only just broaching this movement, and the French tradition on the whole is something to which I have not given serious time. However this seems, for the Fin-de-Siècle, par for the course in terms of content -- adultery, incest, lesbianism, sickness and death, confessions, sacrilege, heresy, all sorts of transgressive, blasphemous content that, while not the most disgusting or straight-forward in its portrayal (Bloy remains better for that), is still unflinching. The only obscurance we receive is from the failing light (these things almost all happen in the evening, interestingly enough) as our narrator struggles to capture every moment. The views are not gratuitous, not for their own sake, but rather seek to record and find the bottom of every experience for man; Barbusse, it seems, would like to find (or already has) an answer for all of humanity, and so must encompass all that is human.

Nobody, before me, has ever been able to see a first glance. I was beside them, but far away from them. I understood and contemplated, without being involved in the excitement of the action or lost in the emotions. That is why I saw that glance. They, for their part, don't know when it began, don't know that it was the first; afterwards, they will forget; the urgent progress of their hearts will destroy those preliminaries. Nobody can know his first glance any more than he can know his last.
I shall remember, when they no longer remember.

To see in his biography that Barbusse began as a poet is not at all a surprise. His style is in-line with some other works from this period, highly allusive and symbolic, occupied with the meeting of the banal and profound in a way not unlike other works in this period. Painterly is a fair way to describe it, given his emphasis on colour and tenebrism, the forms of bodies and cameos. His turns of phrase, even in translation. come off fluidly, and while stylised are not excessive nor overbearing; at times perverse, erotic, yes, but that is the point.
The structure was something as well that caught me: each chapter is subdivided into further short instances that can almost be read as discreet parts, whole thoughts. With Barbusse's deliberate rhythm, and some lapses into higher poetic mode at points, the combined impression reshapes the form of the novel into something more like a string of prose poems. The impression is stronger in some parts than others, of course, but it's very freeing to see the typical formal tensions of the novel blurred in this way; at the very least, it's something that was refreshing to me after the past few things I've read.

The novel is at its best when focused on the psychology and sensation of the voyeur himself -- an unremarkable man recently moved to Paris -- when his pleasures, his sensations, his attempts at a sort of empathy via his peephole, are all forefronted, and he struggles for a problematic sort of "objectivity", to revive his deadened self or even escape his human bounds. Towards the middle, for a span of maybe 50 pages, it leads to those whom he observes speaking more outright, even to political commentary which comes off as rather heavy-handed (the discourse on other lofty subjects, poetry, happiness, memory, thankfully, do not see this same issue, I feel); even if portions of these discussions could be seen as ironic, the central premise of the novel, throughout, is that since the occupants are speaking "unobserved", they are speaking a truth that is unfettered and real -- that is to say, in these portions the work falls away from a real exploration to something more didactic. The struggle between perverse, chauvinistic subjectivity and poetic, celestial objectivity which marks the voyeur's narration is sort of done away with, and becomes polemical, as his occupants speak at greater length. For the project of encompassing humanity, though, if you read the work in that way, it is necessary to include such aspects of life.
As well, the existential conclusion towards which these thoughts lead the narrator, towards an unshakeable sort of solipsism, and the idea that his "hobby" and its fruits could "irrefutably" lead to such a conclusion, are quite intriguing, and so perhaps these ends justify the means. Perhaps, too, it could present a further move towards the interior, an eradication of the voyeur in some regard, as he is lost in the words of his subjects, etc. etc., thematically interesting, yes, all of that, but the sensation of the voyeur is precisely that for which Barbusse seems to have the greatest appreciation, and so to have it pressed aside in favour of something less compelling is a little frustrating, -- even if "necessary" for development. Simply it could have been handled more skillfully. It is conspicuous too that these "truths" in question fall directly in line with Barbusse's own beliefs. The author very clearly had a mission in writing this, something not uncommon for the Symbolists -- and one could say the same thing for other authors; reminds me almost of Tolstoy's narration, the "voice of God" that resounds at points, but given this is a quarter the length of a Tolstoy novel, one notices it all the more. These less compelling portions are the minority of the work, though, at least as I found it, and were not enough to give me an unfavourable impression, thanks to the motion of Barbusse's style.

I am alone tonight. I am sitting at my table. My lamp is buzzing like summer over the fields. I raise my eyes. The stars are pushing the sky away above me, the city plunges down at my feet, the horizon flees eternally from me on every side. The shadows and the lights form an infinite sphere, since I am here.

These evenings I am ill at ease; a vast anguish has taken hold of me. I sat down on this chair as if I were falling. As on the first day, I look towards the mirror, attracted to myself; I examine my reflexion, and as on the first day, a single word springs to my lips: 'Me!'

Still some days after finishing the novel, I am fascinated by the narrative conceit of the Peeping Tom, fixed on this single room whose occupants change day after day. This single idea is, to me, the strongest point of this work, and its implications are all quite well explored.
The most direct way (I think) to read this voyeur's position is a likeness to God. This "divinity" is frequently referenced, the voyeur acting as Eye of Providence in a banal, filthy sense; there are even outright combats with the idea of Deity, religious language and imagery employed in describing all subjects, the narrator himself included (interestingly, I think the procession of tenants can be viewed (in a more highly symbolic way) as a repetition of the Book of Genesis (the activities of one set of tenants are outright described as "Edenic") and the progress of the soul; I don't wish to spoil too much but if you read perhaps you'll see what I mean); all of this creates a nice tension between the narrator's atheism and his addictive habit, an exploration of authority and reality, objective and subjective as mentioned, that sort of thing. There is even flirtation with some Eastern thought, and a passage or two that seems to mirror the Buddhist meditations on corpse decay, although these Oriental influences are never named as such. The struggle against banality, towards profundity, marks the narrator's perversion, paradoxically enough.

More than that, the idea of the novel and the sensations of the voyeur struck me as terribly relevant in the age of social media / surveillance. The voyeur's life consists solely in a vicarious manner; his own life has fallen away, but those whom he watches are, in their habits, their words, open completely in their presumed solitude; their unaffected tenderness, their bodies, the premier moments of their loves, all the things he has lost when regarding himself, become real when regarding the others. His desire, woefully, is the source of his happiness; were it achieved, he would be nothing (there is some poem or aphorism that better says this same thing, only I forget it now). This want for the experience of all lives, all existence, to the point of extinguishing one's own, since the observed reality is somehow the more real; this is (unfortunately) a familiar desire , and close to how I myself have felt at times regarding the digital and the struggles it brings. Not only for the mundane senses, the happy little things, but for the pornographic, the gratuitously morbid, etc. The exact personalities or nature of the events find little importance, only that the ones observed are truly separate from himself, are an other. I don't know if I can develop the point much further, but it's interesting to see that same sensation at root here, but rendered tangible (from a phone to a wall with a hole) and more perverse (in that the vantage is fixed upon this single room, in that the voyeur is separate but truly proximal to his gaze's objects). The connection to the novel's title with this single hole (Hell is conspicuous; it is Heaven that is called into question), the contrast of interior vs exterior (in person and space), light and dark, memory and present moment -- you get the impression. Fun stuff.

I've only read this once and skimmed it over again, so my points are rag-tag. This was an enjoyable, and fairly brief, read. Despite the page count it passed quickly. My edition by Turtle Point Press (the purple cover) has a few spelling errors, and does not make explicit reference in a translator's note about tous or vous, except in one relevant passage. It might just not be an issue for this novel, but it's a gripe I have with most translated works (someday I'll nut up and learn French). Such a keen regard for human sensation and vantage, and this false sense of "closeness", make it quite a relevant work in this era, and provocative in many ways.