This is the first book in the Engines of Light series.
The chapters alternate (perhaps "jump back and forth") between a planet in another star system 500 or so years from now, and Earth in 2048 when information comes out that those on a human space station have been communicating with advanced aliens.
On Earth in 2048, Europe is under some kind of "socialist" rule. There's a ruling party which give some feeling of Soviet bureaucracy, but apparently is not as dictatorial. There was a third world war. Apparently, it was at the end of that war that the Russian army played a role in making Europe "socialist". Not only is Russia involved, but the Russian authorities and European government has some aspects reminiscent of Soviet Communist authorities. Yet, the book was published in 2000 - about 10 years after the end of Soviet Communism. So it seems as if I'm missing something. Perhaps, MacLeod just figures such things happened towards the end of the first two world wars, so why not after the third?
We get a mixed feel of civil liberties in Europe. There are indications they are restricted. But we also see an organization of internet professionals which has its own public building, and the organization operates in some ways inconsistent with what the government would like. We're also told citizens are expected to have guns, partly as a last resort against unpopular governments. We're told the government is not opposed by most people.
One of the internet professionals is in a relationship with a woman who turns out to be a spy working for an anti-socialist group (to some degree associated with the US). The professional gets a data storage disk from her. The disk apparently provides information the aliens sent to the socialist government on how to build an almost-light-speed starship. When the police show up, the professional escapes with the disk and is smuggled onto a boat going to the US in order to avoid being jailed.
Apparently, this is the starship design responsible for those humans being in other star systems 500 years from now. Those humans live on a planet with non-humans. There are a couple of humanoid races and a reptilian race. There is also a squid-related race which humans have limited contact with. There is a more advanced race which is known to have played a role with these other races, but humans don't really interact with them. Two central human characters, Gregor and Elizabeth, work with one of the reptilian people.
Gregor and Elizabeth's society is descended from the European socialist's space station which had the initial contact with aliens. Their area is visited by traders from a colony descended from the capitalist US. The question of further interstellar travel under the control of humans arises.
While there's more threads and details to the book, that's the essence. We're given two levels of background for adventures to come in the other two books in the series. The 2048 chapters give us background for the 500 years from now chapters. The 500 years from now chapters give us what is apparently background for what is being worked towards, but not implemented in Cosmonaut Keep. In that sense, there's some sense of being left hanging at the end of the book. Of course, endings are tricky. There are some one-book novels that will leave you wondering what comes next no matter at which point the author types "The End". So, Cosmonaut Keep could be read by itself, but you may feel as I did that it's largely background leading up to something else. It suggests the series has potential. I just haven't read the other books to be able to say what is made of that potential.
I did feel it took a ways into Cosmonaut Keep for things to clarify - for the connection between the alternating chapters to solidify and for the events 500 years from now to become more meaningful.