The Black Nebula — Robert E. Vardeman

Some days it doesn't pay to get out of your command chair. The Death Fleet is on course for Earth, and Pier Norlin and the crew of the Preceptor are determined to stop them. True to his duty as an imperial officer, even if a self-appointed one, Norlin is determined to carry the warning of impending invasion to Emperor Arian. The reality, though, is that the empire is corrupt and the emperor is a perverted, sadistic madman. Hailed as a conquering hero after he survives one of Arian's "war games," Norlin discovers he's a target for both those determined to stay in power and the factions seeking to overthrow the government.