Thousands of years in the future, humanity has gone extinct (multiple times). It turns out that the human body, while well adapted to survive on earth, is not very well adapted for space travel. But our story follows beings that are so closely patterned after humans that they have maintained much of our social and cultural norms, as well as the physical form factors (though those are often adapted to their environments as well, as we see in the course of this story). Reproduction seems much simpler, with children being "instantiated" rather than born, among other such terminology derived from software and hardware, but on the other hand, messy human characteristics like emotions and love still seem to exist. These are post-humans, but ones that are easy to relate to.
Krina Alizond is a "forensic accountant" who has embarked on a decades long study pilgrimage to visit and work with several of her colleagues. As the book opens, she has just found out that one of her colleagues, a distant sib named Ana, has disappeared in mysterious circumstances. It doesn't take long for Krina to get entangled in a similar web as she seeks out her sister and gradually finds out that the subject of their study, various interstellar debt scams, has become somewhat more complicated in that they've stumbled upon the biggest financial scam in the history of the universe. Along for the ride are a small crew of down-on-their-luck religious folk (though their religion appears to surround bringing back "fragile" human beings), a look-alike assassin hot on Krina's tale, privateers chasing Krina's sister's life-insurance policy, and a group of squid-like communists.
The story generally takes the form of a roller-coaster space opera, with occasional interludes of exposition and info-dumping. It's mostly written from the first perspective of Krina, and you could argue that this is a rather clunky way of delivering a lot of exposition, but on the other hand, I rather thought those chapters were among the most interesting in the book.
Much of the detail surrounds the nature of metahumanity's expansion to the stars, how expensive that proposition is, and how that expense is structured. A lot of this falls back on physical realities like the speed of light, and how it takes centuries to reach new star systems and so on and so forth. It turns out that using something as malleable and fragile as "cash" in that context is rather foolish. Stross takes the unglamorous nature of space travel and expansion and combines it with economics, devising a clever ponzi scheme of debt based on something called "Slow Money":
Slow money is a medium of exchange designed to outlast the rise and fall of civilizations. It is the currency of world-builders, running on an engine of debt that can only be repaid by the formation of new interstellar colonies, passing the liability ever onward into the deep future ...For all its benefits, Slow Money does seem to have a lot of structural difficulties. Krina, being a forensic accountant, has studied all sorts of scams and just plain failures (where, for example, one of the three involved parties dies to soon or other such similarities). Her job is often to swoop in and grab unclaimed slow money that has been sitting around for centuries. There's even a fantastic riff on the usual hand-wavey FTL mumbo-jumbo that always shows up in Space Operas. Here, Stross has adapted the Spanish Prisoner scheme to work on an interstellar scale. If FTL travel was invented, it would suddenly make it possible to trade fast money across interstellar distances, thus depreciating all of the slow money out there (and slow money is what the rich and powerful trade in, so you can see why they would want to preserve the status quo). It's impossible to travel faster than light, but that doesn't stop con artists from attempting to defraud folks (only now it's happening across the vast distances of interstellar space).
By design, the slow money system is permanently balanced on the edge of a liquidity crisis, for every exchange between two beacons must be cryptographically signed by a third-party bank in another star system: It takes years to settle a transaction. It's theft-proof, too - for each bitcoin is cryptographically signed by the mind of its owner, stored in one of their slots. Your slow money assets are, in a very real manner, an aspect of your identity.
...the very slowness of slow money guarantees that it isn't vulnerable to bubbles and depressions and turbulence and the collapse of any currency that is limited to a single star system.
I won't obsess over all the nooks and crannies of these shenanigans, except to say that I really enjoyed the way Stross was able to structure all of this and build his plot around it. I also enjoyed the way that Stross was able to adhere to known physics of the universe and pull interesting story aspects out of that, rather than just hand waving his way around the science the way a lot of space operas would. One example of that is the concept of Slow Money, but we also touch on things like interstellar warfare ("It is a well-understood truism that interstellar warfare is impossible") and some impressive underwater conceptualization.
From what I've read of the Best Novel ballot thus far, this seems to be vying for my number one vote (along with Ancillary Justice). I have not finished Mira Grant's Parasite and have only just begun Larry Correia's Grimnoir Chronicles, but from what I've read so far, I don't see them overtaking Neptune's Brood or Ancillary Justice.
As for the politics of the story, there were maybe one or two paragraphs in the book where it felt like Stross was simply lecturing for ideology's sake, but even those were generally part of the story he was trying to tell here. If I felt obliged to not read anything I thought I might disagree with (or to denounce such things), I'd find myself reading very little (or enjoying even less of what I read). While an author's politics will no doubt color their work, some authors are more difficult to figure out than others. Stross seems pretty easy to read. While I tended to think of his story as a grand thought experiment, there are definitely times (the aforementioned "lecturing" graphs) when I caught a whiff of something more politically motivated. Other authors may be more difficult to suss out, but even a cursory glance at Charlie's Diary indicates that yes, Charlie and I would probably disagree about a lot of things when it comes to economics. And you know what? That's awesome! No one has all the answers, and the idea that we all thought the same things and didn't question anything would be far more terrifying than the fact that Charlie and I might disagree about something (which is not, in any way, terrifying to me).
This might seem obvious, but the amount of vitriol expended over this year's ballot (from both the right and left) seems rather misplaced. When people talk about politics ruining SF, I don't think they're talking about the fact that, for example, Stross's book takes a dim view of capitalism. They're talking about the way we discuss those views, and the fact that some folks are attempting to game the nominations process (and others, seeing this, are attempting to counter by blanket voting against certain authors and works, etc... without even reading those books). Me, I don't want to politicize my every action, I don't want to let politics determine my every move, and I don't want to do an exhaustive biography of every author I could potentially read to see if their views align with mine. Not only is that unnecessary, it's unhealthy. It's a good thing to have your core, foundational beliefs challenged from time to time. It can be infuriating, but it is often productive. There are extreme cases, times when it becomes impossible to separate the art from the artist (hello Vox Day), but I would argue that those should be rare exceptions. Otherwise, I would have stopped reading this book and missed out on what may be my favorite nominee. I used to think this was an obvious truism, but apparently it's not: it's possible to enjoy or like a book without agreeing with it (or without liking the author as a person, not that we ever really get to know the author). I'm really happy that I read this book, and I'm going to be facing a difficult decision when it comes to voting time...