The Ghost Brigades is not a direct sequel to Old Man’s War; some of the minor characters from Old Man’s War are present as is Jane Sagan, but mostly we are following the story of Jared Dirac. Jared is created as a modified clone of a traitor so the army can attempt to transfer a recording ofthe traitor’s consciousness. The experiment fails so Jared is sent off to be a solider in the Ghost Brigades. As his brain grows in experience, the traitor’s memories begin to surface.
The Ghost Brigades deals with how the brain works, memory, consciousness and the importance of individual choice. It doesn’t get bogged down in this, and can be read on the surface level as an attempt to prevent three races from forming an alliance against humanity, and tracking down the traitor who is helping them. There is a moment in the middle of the book where I was sickened (along with the soldiers performing the task) at what they were asked to do. Towards the end of The Ghost Brigades there is speculation about the true motives of the army, with the implication that the political situation is a lot more complicated than we (and the characters) have assumed so far.
There is a novella, Sagan’s Diary, that is a lead up to the next book The Last Colony. This is available on-line so should tide me over until I can get The Last Colony from the library.