Slaves of War — Allen L. Wold

When Anthony Gray, investigative reporter, sets out to discover why so many homeless people have been going missing, he doesn't expect to be captured by aliens. Only his synesthesia prevents the aliens' mind-wipe from turning him into a living brain-slave. His new masters, unaware of his preserved capability for independant thought, install him into ever-more powerful warships, engaged in a war between four very different races. The war has been going on for generations -- nobody remembers how long -- or who started it, or why. Tony develops ever-increasing mastery of all sizes of battlecraft, from one-man scouts to super-dreadnoughts the size of a small city, and acquires brain implants which give him power his masters did not anticipate. Conducting the war has impoverished the home worlds and colonies of all the races, and they are desperate to bring it to a conclusion. Will one race win and subjugate the others? Will Earth, the source of the brain-slaves without which the war cannot be waged, be scorched from existence? Or will Tony Gray find another way out?