OVER BY the west wall, people are killing one another - as unobtrusively as they can manage. Across the room, Paul Maud'Dib is learning how to tame a sandworm, Roderick McBan is amassing the fortune which will enable him to buy a planet, and Charlie Gordon is worried, seeing his own end foreshadowed in the death of a formerly intelligent mouse named Algernon.

You can't see or hear any of this without making a special effort, but if you wander into the Moonstone Bookcellars on Pennsylvania Avenue at the right moment, you may catch owner Phil Grossfield on the telephone having a hard time with a publisher: "We didn't send those books back because we couldn't sell them; we sent them back because we didn't order them. We get cookbooks, about cats, we get all kinds of nonsense. We are a science-fiction and mystery bookstore. We don't sell anything else."

Grossfield's description is substantially accurate but slightly incomplete. If you want a copy of "Roots," you can find it at Moonstone along with other paperback best sellers, and they also deal in fantasy and a little bit of science fact. Grossfield will tell you proudly that the president of Houghton Mifflin said this little bookstore, hidden in a basement under a barber shop near Washington Circle, has sold more copies of "The Silmarillion" than any other bookstore in the United States.

The stock also includes science fiction and fantasy magazines, Moonstone T-shirts featuring the store's emblem (a dragon sitting on a small planet or satellite, reading a book), and, curiously, a small but choice selection of how-to books for would-be writers. Both of the people who work with Grossfield, store manager Steve Brown and part-timer Paul Halpine, are aspiring young science-fiction writers - and so, they have discovered, are a surprising percentage of the store's regular customers.

Grossfield, a Baltimorean, became unemployed two years ago when his employers went out of business, and began Moonstone at an age when most men are ready to retire. "My son Mike owns a general bookstore out in Towson, and he set me up here. Otherwise I'd be in a rocking chair. I don't have to come here every day, Steve and Paul take good care of the place, but I come because I want to. It's like going to a symphony every day. Two years ago, I was 65, now I'm in my mid-50s."

Moonstone is not quite unique; there are somewhat larger stores with a similar specialization in New York and Los Angeles. But it is unusual enough to attract a distinctive clientele which Grossfield describes with obvious pride. "We get the upper 5 percent of the intelligent people of Washington - people from NASA and practically every physicist in this town; we get doctors, lawyers, the upper brackets of government employes. I have never met such a fine, interesting group of people."

Any fiction bookstore is more than a place where commodities are bought and sold; it is storehouse of creative energies - cowboys, pirates, fragile Southern belles or resourceful espionage agents, all waiting quietly for the quickening touch of a reader's eye to bring them to life. But the Moonstone walls contain specialized varieties of encoded energy: tough private eyes and smooth, thoughtful sleuths; space cowboys and mad scientists, galactic empires and curious new concepts in almost any area of human knowledge. Detective stories and science fiction were once dismissed derisively as "pulps" on the theory that the cheap paper they were printed on reflected their true literary value. Today, They have come into their own; they are the subjects of countless university courses and critical studies of the their authors occupy the same library shelves as books about Chaucer and Milton. It is a good time to operate a store devoted to these specialties, but their new-found respectability is only part of the reason. More important is the fact that mysteries and science fiction attract a special kind of devotee - bright, imaginative and above all doggedly devoted to their favorite type of literary narcotic.

"The same people keep coming back all the time," Grossfield says. "It's like a drug I sell them, but it's better than heroin; no bad side-effects."

According to Steve Brown, an average of 40 to 50 customers per day trek down the hard-to-find staircase to the small cellar where Moonstone is located; there may be more on Sundays, a special day when some of the regulars who can't get in on weekdays come around to browse and chat and munch the cookies that some enthusiastic customers bake for Grossfield. Lulls wihtout at least one customer in the store seldom last more than five minutes.

Brown says the regular customers are mostly "scientists or dreamers . . . people with imaginations. Most of them stop and talk, ask 'Have you read this?' or discuss their favorite authors. Science fiction fans have a strong common bond; so do mystery readers. They will strike up conversations with one another. I think a regular bookstore is something like a supermarket; people come in, buy what they want, and go out. Here, the atmosphere is much more like a club. I live in Baltimore, but before I began working here I used to come down to browse as often as I could manage."

So do a lot of other people - and a small sampling supports Grossfield's claim that they are more interesting than the clientele you might meet in the average shoe mart or liquor store. One of the customers who wandered in while Grossfield was being interviewed was Bevan M. French, NASA's program chiel for Extraterrestrial Materials Research and author of "The Moon Book," (Penguin, $4.95), the first book written for lay readers by a top NASA scientist on what was found on the moon and brought back to earth by our astronauts.

"I've been coming in here ever since it's been open," he said.

Presumably, he finds it even more enjoyable to browse the store now that his own book is one of the fast-moving items in its stock. He promised to come in again and autograph some copies if they could be kept around long enough; at the moment, they were out ot stock and more copies were on order.

With French, Grossfield, Steve Brown and various casual browsers all participating, the afternoon developed into an impromptu symposium on science fiction: why all of the noteworthy new writers in the field are women (one of the best, who writes under the name of James Tiptree, is from the Washington area) and how the literary quality in the field has risen in recent years to match the perennially high level of ideas and imagination. Then on to what is and what is not science fiction. Moonstore stocks all the books of Kurt Vonnegut, for example; he began his writing career with a hardcore sf novel called "The Sirens of Titan," and Steve Brown insits that he is still a science-fiction writer, "no matter what he says." He added that in another bookstore he once saw Vonnegut's novel "Breakfast of Champions" shelved among books about food.

Quite a few science-fiction writers do write in other fields, and if their fans demand it, their other books can be found at Moonstone. Anne McCaffrey, for example, is best known for her books (sf, perhaps with elements of fantasy) about dragons, but you can also find her Gothic novels at Moonstone. Isaac Asimov probably comes in first as the author who takes up the most shelf space, but it's hard to tell; he has a whole shelf in the science-fiction section (considerably less than the two shelves devoted to Michael Moorcock), but he also turns up elsewhere in the store as an author of detective fiction, an editor of anthologies, a writer of nonfiction and the author of introductions to books by many other writers.

Most of the store's stock (Grossfield estimates that he carries about 10,000 titles, approximately 75 percent science fiction) comes from the big paperback publishers with a speical interest in the field: Ballantine, Ace, Berkley, etc., but the store also carries a lot of titles by small publishers almost unknown except to fans - a complete set of Arkham House titles, for example, and such highly specialized interests as "Science Fiction and Fantasy Pseudomyms," written by Barry McGhan and published by something called the Misfit Press. Grossfield claims proudly that they do business with every publisher, large or small, who prints science fiction. Do they have every English-language science-fiction title in print?"We try," says Grossfield. "It's a hard struggle," says Brown. If they don't have it and it can be had, they'll get it for your.

For Phil Grossfield, the store's attraction is as much in the customers as in the books; he loves to tell about the customer who went on vacation and let him use his handily located apartment for three weeks, or the one who volunteered to come in and work on Sundays in return for a few free books because he likes the atmosphere. Or the one . . . but let him tell it:

"Six months ago, a young many came inwith his girl friend and picked out some books. I checked them out, and it came to $3.15; then he started fishing through his pockets and became very embarrassed. He didn't have any money with him. He started to put the books back, but I told him, "Take them; pay me next week." And he did; he came back with the $3.15 and asked me: 'Do you have an empty carton?' I gave him one and he went around the store, beginning with A and all the way to Z, and he bought $168 wort of books. 'I'm going to Madrid for two years,' he told me, 'and I don't want to be caught without a supply.'"

Or the one about the man who "came in one day and said, 'I'd like something to read; can you recommend something?' I knew he was a doctor, and we had this interesting new book by Carl Sagan, "The Dragons of Eden," about the evolution of the human mind. I showed it to him, and he said, 'I don't come in here looking for reality; I want you to help me escape from reality for a couple of days.'"

One may suspect that for its proprietor, too, the Moonstone is sometimes a place to escape reality. At present, the store has a mailing list of 2,500 customers, and with some added effort it could probably develop a lucrative mail-order business. But at 67 (although he hopes to keep running the store until he is 90), Phil Grossfield is not interested in building an empire. "All I want," he said, concluding our interview, "is to break even and have a place to go to."

He could do worse. He offered me a part-time job as I was leaving, and I was tempted.