This is the sequel to The Three Body Problem, and apparently there's a third book to come, although the ending of this one didn't feel unresolved. Definitely read the first book first. In this sequel, humanity is aware that Trisolaris has a powerful colonization fleet on the way to earth. They can watch the fleet's progress, and know they have about 400 years to prepare. However, Trisolaris being vastly more technologically advanced than earth has sent Sophons ahead of the fleet to prevent any significant technological advancement of humanity in the mean time. Humans can watch their doom coming, but are stuck with a technology basically what we have now and no hope of improving it.

The book looks at the many different reactions to this - some factions decide to worship the arriving aliens as gods and try to help them, welcoming the end of humanity. Some sink into despair. Some cling to denial. The planetary defense council learns that the one advantage humans have over Trisolarans is the ability to lie - as Trisolarans communicate by direct thought waves, they are incapable of deception or even truly understanding it. The council initiates the Wallfacer project; a handful of outstanding individuals are appointed to the position of Wallfacers, given access to near unlimited funds and resources, and told to come up with and implement plans to thwart the Trisolarans, but without revealing the details to anyone. They must disguise their actual plans so well that even the other humans implementing them don't know them, since any method of human communication can be monitored by the Sophons.

The book basically revolves around the different attempts to save humanity, by Wallfacers and others. It's really a book about ideas and concepts; the characters aren't particularly strong and often quite unlikeable (which makes the first half of the book a bit of a struggle) and also almost entirely male - the author doesn't appear to have any use for female characters except as motivations or foils for the men. Many of the ideas and views of humanity are also quite dark and depressing. However, props to the author for tackling head on the question of why we haven't made contact with any aliens yet, and drawing an extremely logical and believable conclusion.