THE POST-HOLOCAUST novel has been with us for a long time now, ever since the notion of atomic holocaust crawled into our collective unconscious. David Brin's The Postman (Bantam/Spectra, $15.95) is set in the wilderness of Oregon some 16 years after the fateful attack, which combined both nuclear and biological forms of warfare. However, where other, lesser novels have taken those simple beginnings and turned them into an excuse for having giant carnivorous bugs chase hapless heroes across glowing pink deserts, Brin has dispensed with such idiocy and written a fast-paced but thoughtful novel.

The plot focuses on a wandering storyteller who one day puts on the uniform of a dead postman and discovers that he is welcomed as a symbol of civilization. Gradually he starts to become what he plays. But a thumbnail description of this book can hardly do it justice. I will say that what is at the heart of the story is a concept called the "Big Lie." The Big Lie, as Brin's central character explains, is a technique in which you "just sound like you know what you're talking about -- as if you're citing real facts . . . Those who want an excuse to hate or blame -- those with big but weak egos -- will leap at a simple, neat explanation for the way the world is. Those types will never call you on facts." That this explanation is given by a perpetrator of the Big Lie technique gives the theme an extra poignancy. The Postman abounds with mythic dimension, each incident in some way building upon those before it. Every element of the story returns to haunt the hero until he is burdened like Odysseus by the spirits of the dead.

The novel's two weaknesses are that it begins slightly off-key, not finding its stable voice for some 30 or so pages, and that the ever-present wilderness somehow never seems to develop the same mythic proportions as the characters enfolded in that wilderness; these are very minor points, however, and any person who perseveres through the book's first section will be rewarded for the effort. John Shirley

IN JOHN SHIRLEY'S Eclipse (Bluejay, $8.95), the holocaust occurred in Europe, where a "limited" nuclear conflict has been fought in tandem with a ground war. Most of Western Europe is rubble, ripe for the picking. A group called SA (Second Alliance) intends to take over Europe, as well as a space station/city floating above. What makes the SA most interesting is that they are a military force derived from a sect of religious fundamentalists who have taken under their wings various right-wing and fascist organizations. Shirley makes this a vivid and exciting concept, imagining a loose organization of rebels out to demolish the SA. Where Brin's book is light and skimming, Shirley's is thick with characters, sensaion, and imagery -- for instance, a drinking straw that "lit up with miniature advertisements when it was used, lettering flickering luminous green up and down its length" -- but just as fast and hard to put down. Tanith Lee

TANITH LEE, in her new novel, Days of Grass (DAW, $3.50), also gives us a post-holocaust world; however, in this case the catastrophe has been brought on by the arrival of an alien race who have driven the last vestiges of mankind to burrow into makeshift underground worlds.

Early descriptions of the aliens, called "Spiders," remind one immediately of the martian tripods from Wells' The War of the Worlds. They even scream upon attack, an inhuman shriek that again echoes Wells. The main character in Lee's book is Esther, a young woman who has broken a taboo of her underground domain and ventured up to the surface. Esther is a woman in turmoil, maturing from girl to woman both physically and in spirit, eventually becoming leader of a people for whom she feels nothing, as if they and not the unseen beings who have taken over her world were the real aliens. The flaw here is that the subterranean background is barely imagined, so that the reader comes away with a sense that, as in a stage play, the sets have all ceased to exist the moment the spotlight leaves them. This vagueness hurts the book, which begs for a hard-edged reality in the first half to offset the phantasmagoric second part, where Esther is taken into the alien city to meet the master of the "Spiders." Days of Grass is a flawed book, and a despairing novel, but at the same time one that sinks deeply into the substance of myth. George R.R. Martin

SKILL IS something that George R.R. Martin has never lacked. When he is on course that skill can generate a fast-paced, tense work such as "Nightflyers," the opening story in Martin's new short fiction collection, Nightflyers (Bluejay, $8.95). This is a novella reminiscentof Agatha Christie or an "old dark house" mystery, set in space. Unlike these, though, it is graphic, nasty, and never lets up. Had this collection of stories maintained this caliber, it would have been a breathtaking achievement; regrettably such is not the case. Nightflyers begins powerfully but then winds down, each story seemingly weaker than the one before it. By the time I reached "Override," I found myself stuck with the premise that the main characters there are "corpse handlers" whose autonomic system and motor skills are connected to dead bodies, which then perform basic manual labor, guided by the handlers, who in their own way are hardly smarter than the zombies. They are being forced off their planet because a stereotypic bad guy has bought up the rights and wants to get rid of them. He doesn't want their kind around there, and I can't say I blame him. I opted to get out ahead of them.

Next came "Nor the Many-Colored Fires of a Star Ring." The story is the lightest in the book. Martin's dark seriousness needed a comic break, but this is nothing more than a padded and fairly obvious one-liner, for all its attempt to unify the sensibilities of art and science. The last story, "A Song for Lya," has won an award, but I will never understand why. It is a tale of morbid obsession, dripping with treacly, weepy "poetic" prose. Of all the stories in the book this is the worst. It is lamentable that a group of stories that begins with such a strong, entertaining burst as "Nightflyers" must fall flat and leave the reader so dissatisfied at its end. Martin's superior, and chilling, "Sandkings" would have been a better choice to round out this collection. Kelly and Kessel

JAMES PATRICK KELLY and John Kessel have done something different with short stories: they have woven them into the fabric of a novel, Freedom Beach (Bluejay, $8.95). The novel centers on a bizarre therapy program for Shaun Reed, and ths become the dreams used for his therapy. Some of these are hilarious -- like "Faustfeathers," in which the Marx Brothers assume major roles in Marlowe's Dr. Faustus; and "The Fossils," which reads like a capricious piece by Stanislaw Lem. Other dreams are bleak, tragic. In one, Reed comes across Emily Bronte living in a walled-up house that embodies the "empty world" of her soul.

Each of the dream stories, taken individually, shines; however, as a novel there is little if any cohesiveness. And when the book's apparent point is finally established, I can't help but wonder if this trip was really necessary: Reed's therapist says to him, "Writing will not make you a better person; it will just tempt you to pretend your faults are virtues." I fear that is what in fact has happened here. Kessel and Kelly have always had strengths as stylists and here their talents as such certainly sparkle; but their project lacks strength of plot to the extent that we are left at the end realizing that Reed is in therapy because he is a self-centered, self- pitying would-be writer who has destroyed virtually everything and everyone around him -- including a wife who commits multiple suicides. It's virtually impossible to care if he's ever cured. Freedom Beach undertakes to say something important about the human spirit, its title suggesting some relationship to the works of J.G. Ballard. But the similarities are superficial, and the hoped-for payoff never arrives.