An impressive sequel to Old Man's War (blogged here). John Scalzi imagines a future where intelligent life is plentiful in our neck of the galaxy; unfortunately, most of it is really pissed off at humanity and not shy about demonstrating it without remorse. Genetic bio-engineering, transfer of consciousness, computer/human interfaces, and nanotechnology are all highly developed, and all developed into awesome and awful war-fighting tools.
This book concentrates on the "Special Forces" of humanity's interstellar soldiers; they are mostly clones, created from the DNA of Earthlings who volunteered for duty, but kicked the bucket before actually getting to serve. (Hence the term "Ghost Brigades") One exception is the hero: Jared Dirac, a clone of an apparent traitor to humanity, scientist Charles Boutin. Dirac gets a download of Boutin's consciousness in an attempt by higher-ups to get clues about what Boutin's up to; this doesn't appear to work, so Dirac goes right into the Special Forces. But then…
Scalzi continues to write wonderfully Heinleinesque prose, in service of a great yarn. I don't read much science fiction any more, but I've gone ahead and ordered the next book in the series (The Last Colony) and it will go near the top of my depressingly tall (and fortunately virtual) to-be-read pile. There was a Last Colony preview at the end of this book, so I know that the hero of Old Man's War, John Perry, will return; he's only mentioned in passing here.
[Update: just noticed, nearly six years after posting, that I misspelled Scalzi's title The Last Colony as The Lost Colony. Don't know why I did that, fixed in 2015.]