A Jungle of Stars — Jack L. Chalker

Paul Carlton Savage died on July 20, 1969, in Vietnam -- but that was only the start of his troubles! Approached by a mysterious entity called The Hunter, Savage was offered immortality in exchange for his services in The Hunter's continuing war against The Bromgrev. Suddenly, Savage found himself pitted against an enemy he had never seen, an enemy who could be anyone, anywhere, at any time . . . an enemy determined to destroy him and all who got in his way. And in this raging intergalactic war between Good and Evil, Savage discovered that he couldn't be sure whose side he was on . . . .