John Scalzi’s Old Man’s War universe now spans four novels, The Last Colony is the third. Of those initial three this is the one that is most a sequel. The first, Old Man’s War and the second, The Ghost Brigades each stand alone. And I think that The Last Colony actually can do so as well, but this is the first where the main characters are both from the previous books. That said, this is still a complete story and may even appeal to readers who may have not been crazy about the previous books.