THE CRASS COMMERCIAL realities of publishing often have more to do with the course of literature than most critics and writers might care to admit. Witness the sad plight of the modern American short story. Once a thriving form, it has been declining precipitously for more than 30 years, and today dances on the edge of extinction.

The responsibility for this sorry state of affairs does not lie with the writers, who are by and large as talented (and untalented) as those of previous generations. It is publishing that has done in the short story. The death of old pulps and the continuing swing toward nonfiction in the slick magazines left very few places for stories to be published. In book form, story collections sold even worse than most first novels, which is to say not at all, and as a consequence they are rarer than $1.95 paperbacks.

Science fiction has proved to be a curious exception to this generally gloomy picture. The reasons, once again, are more commercial than literary. A handful of struggling sf magazines, most of them descendants of the pulps, have persisted to this day, printing hundreds of thousands of words of short fiction every month, and thereby affording an outlet. Moreoever, while it is true inside sf as well as outside that story collections do not sell as well as novels, the difference is markedly less pronounced in sf than in general fiction. Thus the various science-fiction publishers continue to issue a goodly number of roundups and anthologies, by old hands and newcomers alike. Four recent collections give a case in point.

The Last Defender of Camelot (Pocket Books, $2.50) by Roger Zelazny is the pick of this particular litter. Zelazny's career followed what has become a very typical path within science fiction. He began publishing professionally in 1962, with short shories, and it was in the shorter forms that he first made a name for himself. Like many other writers, however, Zelazny was soon seduced away from his first love by the greater glory and riches of the novel. Today his shelves creak beneath the weight of sundry Hugos, Nebulas, and other awards, and he must certainly be regarded as one of the most important contemporary science-fiction writers. Numerous novels have come from his typewriter in the last 15 years, a steady stream of them -- but Zelazny short stories have been relatively few in number.

There has been only one previous gathering, The Doors of His Face, the Lamps of His Mouth, and Other Stories, originally published in 1971. Containing most of Zelazny's classic work at the shorter length, that book ranks among the three best story collections of the last decade and is still worth seeking out.

The Last Defender of Camelot is not so good as that, but it is still good enough, and long overdue. The collection includes several very early short-shorts, some strong material from the '60s that was omitted from the earlier collection for one reason or another, and virtually all of the short stories that Zelazny published during the 1970s. The author has also contributed a brief introduction on each individual story. (Interested readers will find a complete listing of Zelazny's fiction, and criticism thereof, in Joseph L. Sanders recent Roger Zelazny: A Primary and Secondary Bibliography, available from G. K. Hall for $15.)

Some of the earliest material in The Last Defender of Camelot, which includes Zelazny's first published story, is admittedly minor, though it shows flashes of the strong imagery and evocative prose that were to become Zelazny's trademarks. These early stories are also very short, however, so it is not long before the reader hits the later and meatier pieces.

In the interest of buyer protection, it should be mentioned that the book's two longest stories -- "Damnation Alley" and the Nebula Award-winning "He Who Shapes" -- were both later expanded into novels. "He Who Shapes" became The Dream Master, while "Damnation Alley" mutated into a novel (and an absolutely dreadful movie) of the same name. In his notes, Zelazny indicates that he still prefers the original, shorter versions, a judgment I am inclined to agree with, especially in the case of "Damnation Alley," a straightforward adventure story that reads better in its faster, leaner form.

Harlan Ellison gained prominence during the same period as Zelazny, but his career has been very different. Ellison is anything but typical. Instead of starting with short stories and shifting to novels for the greater financial rewards, Ellison published a number of forgettable novels early in his career and then shifted almost entirely to short fiction. He is almost unique in being a major contemporary writer whose reputation rests in its entirety on his short stories and his frequent short-story collections.

Shatterday (Houghton Mifflin, $12.95) is the latest Ellison collection, featuring virtually all of his most recent material plus one older collaboration. Unfortunately, it is a long way from being one of his best.

The problem with being a short-story writer today is that you have to publish several collections to earn as much money as you might on a single strong novel. To make up each new collection, one either must duplicate material from previous collections, or use virtually everything one has written recently. Ellison chooses the latter course, and Shatterday is thus a potpourri of his recent work, good, bad, and indifferent.

When Ellison is writing at the top of his form, he is one of the great living American short story writers. "Shatterday," represents Ellison at his best, a powerfully written, unforgettable fantasy about identity. "Jeffty Is Five," a gentle, moving account of a boy who never grows, is almost as good, and there are a few other strong stories to keep these two company.

Unfortunately, the rest of the book is not up to these standards. Some pieces, like "Django" and "Shoppe Keeper" and "The Man Who Was Heavily Into Revenge," are only minor, but there are others, like "How's the Night Life on Cissalda?," that are embarrassingly bad. Ellison has often raged against the science-fiction label that has haunted his career, preferring to be called just a writer, or at most a fantasist. He has a good point. He is a fantasist, really, and his best stories are set firmly in the here-and-now and partake strongly of the flavor of our times. On occasion he does write a marginal piece that uses some of the devices of sf, and could be so categorized, but almost invariably these are his weakest stories. This is certainly true in Shatterday.

Perhaps the most interesting story in the book -- though not the most successful -- is the longest, "All the Lies That Are My Life," which is not science fiction, and not fantasy, but instead a sort of fictionalized autobiography or roman a clef about the death of a writer named Kercher O. J. Crowstairs, who very much resembles Harlan Ellison, with a few dollops of Robert Silverberg thrown in. Other characters are also based closely on various friends of Ellison. In his introductory note, Ellison stresses, "This is fiction, not personal memoir." This question of the overlap between the writer and the work is raised several other times in the book too; Ellison is clearly unhappy about those who read too much of him into his stories.

It is not a problem he is likely to escape. Ellison is outspoken and charismatic, and his lecture tours, autobiographical story notes, and frequent appearances on talk programs have made him a public figure. As with Norman Mailer or Hunter S. Thompson, it is difficult to read the work now without being constantly aware of the author. This is emphatically so of "All the Lies That Are My Life." Ellison makes it clear in his introduction that he wants the story read as fiction, that he doesn't want the readers to play the game of figuring which parts are real and which are poetic license. Unfortunately, in this case the game provides the chief fascination of the story. Those who know Ellison or think they know Ellison may have a great time picking apart this tale and speculating about it. But read in a vacuum, soley as fiction, it is a flat failure. Kercher O. J. Crowstairs, as a fictional creation, is empty. He lives only as a shade of Harlan Ellison.

Ellison and Zelazny are both major, long-established writers, but it is a sign of the relative health in the sf field that one does not have to be of their stature to get a short story collection published. Collections by two relative newcomers, Orson Scott Card and Barry B. Longyear, underline that point. Card was the winner of the 1978 John W. Campbell Award as the best new writer in the field. Longyear took the same award for 1980. Card has just published his first collection, following a couple of novels. The prolific Longyear has already come out with his second collection.

Of the two, Card's Unaccompanied Sonatra and Other Stories (Dial Press, $10.95) is much the stronger, though it is not in a class with the Zelazny or the Ellison. Card has been the victim of a curious critical whiplash; vastly overblown praise for his first efforts, followed within the blink of an eye by savage critical "reassessments." Neither verdict was justified. The truth is that Card is a young, talented, and ambitious writer who is still in the process of growth and you can see him growing visibly in this volume. The title story, one of his most evocative works, demonstrates how far he has come since the early pieces that open the book, though it restates some of the same themes.

One cautionary note: Unaccompanied Sonata is often grim reading. In a brief afterword, Card says all these stories have happy endings. If so, I pray I am spared Card's sort of happiness. Story after story deals with death, pain, mutilation, dismemberment, all described in graphic detail -- Card is not one to look away. One story features a detailed account of a woman's breats being cut off and eaten. A later tale has for a horror a thing that looks like a thalidomide baby, flippers and all. The protagonist drowns it in a toilet and cuts up the body with a knife. The hero of the title story suffers one mutilation after another. Each of these tales might be powerful, read in isolation. Read one after the other, they are rather much. I am pleased to add that Card seems to be changing in this regard, too. In his more recent work, he seems less obsessed with graphic violence, and what remains of it is handled in much less heavy-handed fashion.

There is no graphic violence in Barry B. Longyear's Circus World (Berkley, $2.25), which is one of those collections disguised as a novel, a series of interconnected tales of the planet Momus, where a traveling circus crashed and established a society. There is also no suspense, no characters, no discernible human emotion, and not much of a plot. In a genre famed for its imaginative outre social systems, Longyear's Momus ranks as one of the least convincing and silliest. Circus World is hard to take seriously, but neither is it funny enough to take humorously. This is the lightest sort of fluff, perhaps capable of passing an hour's worth of time, but afterwards instantly forgettable. Longyear has done somewhat better, more ambitious work elsewhere. Momus is pure cardboard, written -- with one eye on those commercial realities I began with -- to pay the rent. Pass this one up.