Not really but almost a sequel to his first novel (Old Man's War), this one returns to the same universe a few years after those events and share's a few characters.
This novel deals with the other side of the Colonial Defence Force, the Special Forces. They're nicknamed 'the ghost brigades' because they're engineered super-soldiers based on the genetics of people who enlisted but died before they were called up. These people are born into adult bodies and educated largely through their 'brainpal' internal computers.
With all that setup, you can see that this is going to be a story about what it is to be human, the ethics of creating people for an unpleasant job, and where their duty lies. Scalzi addresses these issues thoughtfully and has some interesting takes on parts of it. This also makes it a rather more serious novel than the first, although it's got some humour too.
On top of all that stuff, it's a good military sf novel, with some good action and a backdrop of interstellar black ops and war.