John Scalzi, The Ghost Brigades, Tor, 2006.
This is the sequel to Old Man's War. It expands our understanding of the world presented in the first book, and presents a more multi-layered perspective.
On one level, it echos part of the story of Old Man's War, which described the training and combat activities of a CDF soldier. The story this time describes the origins, training and combat activities of a Special Forces soldier. To me, it was immediately obvious what the term "Ghost Brigades" meant when it was first mentioned in OMW. This level of the story provides a first-person report of what that experience is like.
On another level, the main character is not, or not entirely, what he believes himself to be. He has the memories of another person, which take a while to emerge into his consciousness. He finds that his whole existence is aimed at discovering these hidden memories and acting upon them.
Despite his unenviable origins, Jared Dirac becomes an interesting person in his own right, and serves his purpose in an unexpected way. The memories that rise up in him are those of a scientist, Charles Boutin, who has turned against the Colonial Union and plotted with other races. Unfortunately, he comes across as a caricature of a mad scientist type villain, despite efforts to explain his choices.
This is a story about identity and humanity. Can people created as super-human fighters, trained from inception for warfare and survival, become fully human? The answer here is a resounding "yes".