This is not a review of Nethereal, and I will try not to spoil too much of the plot. I am going to focus on an area of the book that I believe stands out above all the others: the setting and its worldbuilding, and what makes them unique.
Building a whole fictional world for a fantasy or science fiction book is a staple (if not a necessity) of the genre, but I think the correct label for what happens in Nethereal would not be simple worldbuilding but something like a "Pocket Universe." In other books a few kingdoms, a continent, perhaps a planet or even a galaxy are described as background. If the metaphysical aspects of that creation are explained at all, they are merely hinted, waved away as "mythology", or just another dimension for the protagonists to stroll about. But the origin, ending, and limits of those fictional universes are left murky enough, or they may not be relevant to the story at all.
The world of Nethereal, on the other hand, is defined and bounded physically, temporally, and -I'd say- spiritually. And those limits are not background fluff, they are directly linked to the plot of the book and what the protagonists do, see, and suffer. And talking about limits, it's also a small universe, and perhaps because it is implied that the whole material universe (The Middle Stratum) is under the control of The Guild (something like a non-mutant version of Dune's navigators,) there doesn's seem to be many worlds (or "Spheres") unclaimed or ready to be discovered (and, therefore, worthy of being described.)
That is, in fact, one of the reason behind the expedition the protagonists join: to discover new worlds unclaimed by the monopolistic Guild. But where would someone discover new frontiers in a universe without any unclaimed lands? In other non-mundane dimensions. And because this book has been universally described as "space pirates in Hell," I'm sure you can guess where those dimensions end up being. Furthermore, unlike other space operas, there are no bizarre alien species, weird fauna, and so on. Hence the "Pocket" bit I mentioned since it feels like a small (or compressed?) -physically and thematically-Universe.
Temporally, the world of Nethereal also seems to be bounded, although there are hints of something much bigger waiting behind, including a brief reference to our own universe (the Judeo-Christian creation myths, to be precise) towards the end. Both its beginning and possible ending are part of the story, and the protagonists end up walking on the -if you can call it that way- ground where the universe was first created. They also met some of the gods or entities involved in that cosmic drama, so it's not like they are just following mythologies or prophesies coming from the mouth of a raving priest. For example, at the end of the book, one of the protagonists explains (as much as these things can be explained) how the universe was created and how it may be destroyed. And they are not listening to someone teaching them a theology lesson; it's a very real and immediate threat.
I think now you may understand why I described it as a Pocked Universe. The whole worldbuilding process left me with the impression of an object that you can see, hold, and manipulate, almost as if it were a well-defined toy. In fact, that's somewhat similar to what the villain of the story believes, but I will come back to that later.
While in most books the origins of "magic" are usually left unexplained, in Nethereal they are precisely defined (again, as much as these things can be defined.) Magic is an intrinsic part of that universe, although it is not named that way and the official worldview is a materialistic one; they understand those powers as equations, a form of mental discipline, and energy manipulation. And "energy manipulation" is probably the best description of magic in Nethereal considering its most obvious use is powering and running the spaceships (or ether-runners,) most of them controlled by The Guild. "Magic" is the manipulation and consumption of prana (yes, like the Hindu concept) through Workings (or spells,) and because prana is also the vitalistic principle and the basic building blocks of the Universe, Magic is essential to its structure and to Nethereal's plot. In other words, Magic is not something outside of ordinary reality, something to weave a few miracles and transform people into frogs, or to perform a few pyrotechnic displays and make combat easier (although there is a quite a lot of these,) but an integral part of the laws of the universe. In fact, it is stated that prana is running low and that the Universe may be dying (e.g. whole species are disappearing.) It's not unlike a town whose well is running low.
That word, Well, is also appropriate because that's how prana's source is described, as the White Well. That is why I said "magic" is directly linked to the workings of that universe because it's nothing more than the manipulation (and exhaustion) of vital forces. That Well is not located in the physical or mundane Universe, but "above" it. For lack of a better description, that energy "flows down" through the Middle Stratum, "loses its potency," and ends up in the lowest (even lower than Hell) Stratum, the Void. That process sets the interplay between good and evil, life and death, creation and destruction, and the forces of entropy. It is also implied (or that's how I understood it,) that this process of universal creation and destruction (including a Deicide) has gone for a long time.
Remember when I said that the villain also saw the universe as some kind of a toy? Well, the exact word he uses is "clock" -and here it is impossible not to spoil a little-, and that's what the cosmically bored villain says when asked about what does he want:
"I wish to pass the temple's threshold, the point from which creation sprang. And beyond that: another universe? Countless others? Nothing at all? None know, though it matters not. I only wish to be quit of a world run down like a neglected clock."
Having said all that, do not mistake this book for the brainy version of a worldbuilding fancy or a theological thought experiment. There are no dump expositions here, and the novel is an action adventure through and through. In fact, I would have enjoyed a few more explanations and descriptions about the inner workings and customs of this or that Circle (my favorite part of the book was the 5th circle of Hell and the description of Despenser's domain.)
Philosophical as they may be, in Nethereal, problems are solved by punching them and sending them back to Hell (where half the book happens, anyway.) And the bigger the predicament, the bigger the punching. If there are stories that are described as "from rags to riches," I guess you could call Nethereal a "from barely being able to pay the bills to punching God (or Satan)" story.