The Ghost Brigades is a science fiction novel, a sequel to author John Scalzi's first novel Old Man's War. That is, it is set in the same fictional universe, but it is not a direct continuation of that novel's plot. While the first novel's protagonist is mentioned, he never appears "on stage". Instead, Scalzi takes one of his most interesting ideas from the original novel and expands greatly upon it.
(Warning: Minor spoilers to Old Man's War follow.)
In the first novel, Scalzi posited human super-soldiers with genetic enhancements and nanotech computers embedded in their brains. Most of these were host bodies for adult humans who had their consciousness transferred into a new, younger, stronger, and faster body. However, some soldiers were allowed to develop on their own, without a transferred adult consciousness. These members of the "ghost brigades" were secondary characters in the first novel, but are the main characters of the sequel. In so doing, Scalzi has effectively inverted the premise of his first book. These soldiers are not the elderly in the bodies of the young, but infants in the bodies of adults. The nanotech computers bootstrap the characters with language and motor skills, turning them into battle-ready soldiers in mere weeks. Hijinks ensue.
Scalzi is in excellent control of his tale. Unlike the previous novel, the plotline isn't borrowed, and a richer story results. He deftly establishes a main character who is a bit outside the norm and can thus explore this world without too much forced exposition. His plot drives forward in a compelling fashion, to a slightly predictable but nonetheless fitting ending. Scalzi explores the ramification of the "on board computers" in our soldier heroes' minds in an interesting and satisfying way.
My main complaint would be that he doesn't make the young soldiers of the "ghost brigades" strange enough. They assimilate 20th century earth culture in their training which allows them to have references that are comfortable to the reader. I can accept that. Yet they just don't seem as weird as their upbringing ought to make them. They're a bit too much like classic SF marines — you could drop them into Clone Trooper armour and deploy them to a Star Wars: Republic Commando novel and they'd be perfectly at home.
But that aside, it's a great read for SF fans. It brings richness to this fictional universe and bodes well for the third entry in the series, The Last Colony, which is due for release on April 27, 2007.