Much of Death's End is told from the perspective of one person, which helps to maintain some continuity across all these changes. That person is Cheng Xin, a spaceflight engineer who becomes an intelligence operative and ends up traveling to future eras via hibernation technology. She's a very empathetic character, quickly identifying with the people and situations she finds when reviving in a new era. That empathy also means she is unable or unwilling to sacrifice others for potentially greater gains, though, and she makes several key decisions through history that have some dire consequences for humanity.
The idea of using more (or less) than three dimensions is a recurring theme throughout the series. The Trisolarian civilization uses multi-dimensional technology to build "sophons" in the first novel, and those play a huge role as instantaneous communication conduits, spies, and agents of sabotage. In Death's End, the idea of regular three-dimensional objects moving into either four- or two-dimensional space plays a large role, and various theories about the structure of the universe imply even higher dimensions. Liu Cixin isn't the first author to delve into the idea of changing dimensionality, but I thought the way he used those ideas was unique and interesting.
One of my favorite parts of Death's End is a series of allegorical fairly-tale stories which are used to communicate direction for scientific research. The Trisolarians are listening in on communications between the one human with access to their knowledge and those in back in the Solar System, so the information can't be stated directly. Trying to decipher the meaning of the stories becomes a puzzle that lasts years. Each hidden meaning that is revealed leads to shifts in the direction of humanity's understanding of science and technology. I enjoyed the mystery aspect of guessing the meaning in the stories, as well as appreciating the literary skill involved in integrating the very different fairy tale style in the middle of a science fiction novel.
The events in Death's End do wrap up most of the questions posed by the earlier novels. The resolutions aren't necessarily great for humanity, though. There are some pretty bleak outcomes for a lot of people. It's not just humans, either - the Trisolarians have serious troubles as well. The threat of the "dark forest" of the universe is very real in this story, and the author doesn't hand out any sort of magical solution to the situation. While the results may not be great for a lot of people in the book, I appreciate this approach. It would have been very easy to give humanity some kind of breakthrough discovery that made them immune to the universal threats, but if he had done so, I think it would have invalidated the entire idea.
It's not all doom and gloom for the end of this trilogy. There's no perfect happy ending for Cheng Xin, or the human and Trisolarian races, but life and hope persists. I found Death's End to be a satisfying conclusion to a grand epic story set in an intriguing universe.
