The Cassini Division was the first Ken MacLeod book to be published here in North America, so I suppose it's strangely appropriate that I'm reading it last, out of all of his so-far published novels.
The Cassini Division is one of the four Fall Revolution novels MacLeod wrote, and is the only one of the four that is a direct sequel. It follows directly on the events of The Stone Canal, in which a pair of uploaded humans on anarcho-capitalist New Mars travelled back down a wormhole path to the Solar System. Those humans - Jonathan Wilde, aka Jay Dub, and his artificial intelligence wife - get a walk on at the beginning of the novel, then vanish. The torch is passed to Ellen May Ngwethu, a 200 year old leader in the titular Cassini Division. The elite of an egalitarian future, the division guards humanity from what is left of a civilization of superhumanly intelligent uploaded minds that fell into the atmosphere of Jupiter during the events of The Stone Canal.
While this kind of thing - superhuman AI and uploaded humans, threats from godlike posthumans, wormhole travel - are relatively familiar to SF fans, the milieu isn't. The Solar Union that Ngwethu fights for is a communist utopia, in which the state really has withered away and humans are prosperous, free and happy.
MacLeod is, by his own admission in many forums and interviews, a long-time socialist and former Trotskyist. He has taken the various criticisms of central planning made by people like Mises very seriously, however, and the Fall Revolution books never shrink from alternate political points of view. Indeed, Ngwethu is probably the least political of his protagonists in the series. Jonathan Wilde, of Stone Canal, is an individualist anarchist shit-disturber, Myra Godwin of The Sky Road is a Marxist politician, and Moh Kohn of The Star Fraction is a Trotskyist himself, living in a libertarian enclave. Along the way in MacLeod's various other books, everything from anarcho-syndicalism to agorism to various forms of townhall democracy show up, often in wild combinations.
The communism of The Cassini Division is presented both very sympathetically - I wouldn't mind living in such a system - and with its few flaws readily apparent. Although 30 billion people live under the Solar Union, there are a fringe of "non-cooperators" or "non-cos" who live in a grubby free-market society on Solar Union fringes. The Solar Union mostly ignores them or co-opts their best people. In a typical American SF novel, the non-cos would be the heroes, fighting against the monolithic Solar Union. Here, it's a lot more complicated. (There are jokes about this, of course. A solid gold statue of Mises stands in the chamber the Solar Union once used for central planning meetings.)
Ngwethu, like Clovis in The Sky Road, is a character who seems to be living in one of the times when humans have convinced themselves that history, per se, is over, and the right side has won. As is always the case, she is proved wrong. The Jovian intelligences have become more active and organized, and Ngwethu is convinced they are a threat. She enlists a famous non-co physicist to try to solve the mystery of the wormhole the Jovians created, then travels through it to the other side. The anarcho-capitalists of New Mars begin to flit back across, looking for new markets - something that is anathema to the Solar Union.
Meanwhile, the Jovians have resumed contact with humans, and claim they are no longer hostile. Ngwethu wants them exterminated anyway, saying beings that think and live a thousand times faster than humans can't be trusted to keep agreements. Only life can hold a real consciousness, she believes, never a machine.
Many reviews of the book have found it dissatisfying, and I've seen it ranked frequently as the least interesting of the Fall Revolution series. I have to disagree. Except for the ending - which seems to go against everything MacLeod was aiming at - I found it very gripping. The milieu is fascinating, and is one that I don't think has been seen much outside of hopelessly utopian political fiction. It was a lot better than The Sky Road, which never really came together as a coherent whole, and which I felt left dangling plot threads all over the place.
On the politics of the book, I'd say that it was long past time that someone started writing science fiction in which socialism matters, whether it's triumphant or simply at the margins. (Kim Stanley Robinson can't do this alone!) I've seen people argue that The Moon Is A Harsh Mistress created more libertarians than Atlas Shrugged ever did, simply because it's a better book, and because it's lessons go down easier with a nice coating of adventure, fun, and likeable characters. Similarly, I'd love to know how many people will find their attitudes to socialism changed by The Cassini Division. The Solar Union is attractive enough that a vast majority of humanity has joined, but it is flexible enough to allow dissidents and outright criminals to avoid it. It has no laws and is fundamentally a huge, voluntary organization. But MacLeod is honest enough to point out the flaws of his vision - it's a bit dull and parochial, and its people carry around prejudices that they don't often recognize as such. The Union also has a "might makes right" approach to solving external problems: there is the unspoken assumption that if the non-cos ever became powerful enough to be a threat, the Union would have few qualms about wiping them out.
The book has also added a few phrases to the Standard SF Quote Lexicon. The most frequently repeated is the one about uploading being "Rapture for the nerds." My favourite, though, is MacLeod's description of humans as "fish wearing space suits."
My last word is: read The Stone Canal first. Despite the fact that it uses MacLeod's dual-track narrative, which drives me crazy, it is the better book of the two. And it will help you understand what happens in the second half of Cassini Division.