From the description:
"Seventy-five-year-old John Perry joins the Colonial Defense Force because he has nothing to keep him on Earth. Suddenly installed in a better-than-new young body, he begins developing loyalty toward his comrades in arms as they battle aliens for habitable planets in a crowded galaxy. As bloody combat experiences pile up, Perry begins wondering whether the slaughter is justified; in short, is being a warrior really a good thing, let alone being human? The definition of "human" keeps expanding as Perry is pushed through a series of mind-stretching revelations. The story obviously resembles such novels as Starship Trooper and Time Enough for Love, but Scalzi is not just recycling classic Heinlein. He's working out new twists, variations that startle even as they satisfy. The novel's tone is right on target, too—sentimentality balanced by hardheaded calculation, know-it-all smugness moderated by innocent wonder."
4.3 stars on 250 reviews, 324 pages.