Well, Polystom is Roberts' exploration of the old twist-in-reality plot. It's basically about a young man named Polystom, a sort of aristocrat, whose grandfather is involved with an ongoing war his people are waging on an enemy planet. After his unsuccesful marriage, Poly decides to try his hand at this war business. Out on the battlefield, in the trenches, he is increasingly forced to confront the ghosts of his past and the very nature of the reality he exists within.
It's beautifully written, abstract in conceit and leaves you with much to ponder on, like all of Roberts' novel. It may well be my personal favourite of all his works.