The amazon.com blurb:
The Ghost Brigades is the sequel to John Scalzi's first novel Old Man's War. That was a book I loved right away, and this one is just as good. Just, read them in order. Otherwise, you'll completely ruin one of the big plot twists from the first book.
The Ghost Brigades are the Special Forces of the Colonial Defense Forces, elite troops created from the DNA of the dead and turned into the perfect soldiers for the CDF's toughest operations. They’re young, they’re fast and strong, and they’re totally without normal human qualms.
The universe is a dangerous place for humanity—and it's about to become far more dangerous. Three races that humans have clashed with before have allied to halt our expansion into space. Their linchpin: the turncoat military scientist Charles Boutin, who knows the CDF’s biggest military secrets. To prevail, the CDF must find out why Boutin did what he did.
Jared Dirac is the only human who can provide answers -- a superhuman hybrid, created from Boutin's DNA, Jared’s brain should be able to access Boutin's electronic memories. But when the memory transplant appears to fail, Jared is given to the Ghost Brigades.
At first, Jared is a perfect soldier, but as Boutin’s memories slowly surface, Jared begins to intuit the reason’s for Boutin’s betrayal. As Jared desperately hunts for his "father," he must also come to grips with his own choices. Time is running out: The alliance is preparing its offensive, and some of them plan worse things than humanity’s mere military defeat…
All of the background that wasn't explained in the first book is given here, although it ends up leaving you with plenty more questions for the next book. Not only that, but the ending is definitely open to sequels, of which I know there are two more (both on my "must read" list too), The Last Colony and Zoe's Tale.
If you like classic science fiction a la Robert Heinlein, you'll love this. There's the same timeless feel and interesting characters. This time the main characters are Jane Sagan and Jared Dirac. Jane Sagan was the special forces character from Old Man's War, and it's definitely neat to see things from her perspective, and to find out more about the CDF special forces way of life.
The Ghost Brigades stands on it's own well, so you don't need to have read Old Man's War first, but the two books go together really well. This book doesn't read like a "middle book" at all, unlike some series I've read. Definitely a five star book. It certainly kept me up past my bed time!