John Scalzi's "The Human Division" (Tor, 431 pages, $25.99), the fifth novel in a sequence that began with "Old Man's War" (2005), is in a long tradition too, back to Robert Heinlein's "Starship Troopers" (1959), Joe Haldeman's "Forever War" (1974) and Orson Scott Card's "Ender's Game" (1985). Humans have colonized other worlds and created a specialized military caste for protection from potentially hostile aliens. Ever since Heinlein, however, there has been uneasiness about the too-quick identification of "alien" as "enemy."
Mr. Scalzi's current volume follows up on that uneasiness. The "division" part of "The Human Division," is between Earth and its colonies. Both sides need each other, but their interests are opposed. Alien races, meanwhile, alarmed at expansion of the Colonial Union, have formed a "Conclave" to block it. But will they distinguish Earth, which supplies recruits for the Colonial Defense Forces in exchange for protection, from the CU?
Unusually, "The Human Division" consists of 13 linked short stories. Showing a complex situation from different angles, they roughly alternate between the main plot and side plots. It's slightly confusing at first, but Mr. Scalzi is one of the slickest writers that sci-fi has ever produced. His clever narration keeps you turning the pages, as the Conclave confronts wildcat colonies, the CU deals with "Earth Rule" insurgents and agents provocateurs unsettle all sides.