FIRE AND RAIN by Oswald Rivera (Four Walls Eight Windows: $17.95; 185 pp.). This novelized account of an actual jail riot in Vietnam is a horror story of the worst kind, recounting the horrors of war graphically, visually and with the utmost cynicism. A grotesquely one-sided battle it is, and the victory of the (mostly white) guards is total. The action, in combat and in the brig, is mostly told from the point of view of a young corporal from Ohio jailed for killing a sergeant. It's told in the crude military lingo of a "grunt" (infantryman). The author, a Vietnam veteran, says he was at Da Nang, not as a prisoner but "as part of a unit pulling guard duty in the area." That he can recount with such force the brutality of the guards as experienced by the prisoners is a tribute to his empathy for the victims. Or is this book an effort to purge the incident from his mind?