I received a letter from a fan the other day, one who had bought a copy of Agent of Byzantium by Harry Turtledove, which appeared in a series entitled "Isaac Asimov Presents." (That's why he wrote to me.)

The cover shows a man dressed, says my correspondent, "in a Romanesque military uniform, holding a Roman helmet in his left hand." He also carried "a very large, very modern, very lethal-looking blaster rifle" and "an electronic scanning device."

My correspondent was intrigued by the anachronism, bought the book, read it, and "enjoyed the book." However, he found no place in the story where a man was holding such a rifle and scanning device, and he felt cheated. He had been lured into buying and reading the book by an inaccurate piece of cover art, and he wrote to complain.

So I thought about it. Now my knowledge of art is so small as to be beneath contempt, so naturally, I can't be learned about it. There is, however, nothing I don't understand about the word trade (fifty years of intimate, continuous and successful practice at it gives me the right to say that), and so I will approach matters from that angle.

I see the reader's complaint as the protest of the "literalist" against "metaphor. " The literalist wants a piece of art (whether word or picture) to be precise and exact with all its information in plain view on the surface. Metaphor, however, (from a Greek word meaning "transfer") converts one piece of information into another analogous one, because the second one is more easily visualizable, more dramatic, more (in short) poetic. However, you have to realize there is a transfer involved and if you're a "born-again literalist," if I may use the phrase, you miss the whole point.

Let's try the Bible, for instance. The children of Israel are wandering in the desert and come to the borders of Canaan. Spies are sent in to see what the situation is and their hearts fail them. They find a people with strong, walled cities; with many elaborate chariots and skilled armies; and with a high technology. They come back and report "all the people we saw in it are men of a great stature. And there we saw the giants...and we were in our own sight as grasshoppers and so we were in their sight." Right! They were of "great stature" in the sense that they had a high technology. They were "giants" of technology and the Israelites were "grasshoppers" in comparison. There was as much chance, the spies felt, of the Israelites defeating the Canaanites as of a grasshopper defeating a man.

It makes perfect metaphoric sense. The use of "giants" and "grasshoppers" is d ramatic and gets across the idea. However, both Jewish and Christian fundamentalists get the vague notion that the Canaanites were two hundred feet tall, so that ordinary human beings were as grasshoppers in comparison. The infliction of literalism on us by fundamentalists who read the Bible without seeing anything but words is one of the great tragedies of history.

Or let's turn to Shakespeare and the tragedy of Macbeth.

Macbeth has just killed Duncan and his hands are bloody and he is himself horror-struck at the deed. Lady Macbeth is concerned over her husband's having been unmanned and gives him some practical advice. "Go," she says, "get some water and wash this filthy witness from your hand."

And Macbeth, his whole mind in disarray, says, "Will all great Neptune's ocean wash this blood clean from my hand? No. This my hand will rather the multitudinous seas incarnadine, making the green one red."

It's a powerful figure, as you see a bloody hand dipped into the ocean and all the vast sea turning red in response, but, literally, it makes no sense. How can a few drops of blood turn the ocean red? All the blood in all the human beings on Earth if poured into the ocean would not change its overall color perceptibly. Macbeth might seem to be indulging in "hyperbole" (an extravagant exaggeration which sometimes makes its point, but usually reduces it to ridicule).

This, however, is not hyperbole, but metaphor. Consider! Macbeth has killed a man who had loved him and loaded him with honors, so he commits the terrible sin of ingratitude. Furthermore, the man he murdered was a guest in his house, so that Macbeth has violated the hallowed and civilized rules of hospitality. Finally, the man he murdered was his king and in Shakespeare's time, a.king was looked upon as the visible representative of God on Earth. This triple crime has loaded Macbeth's soul with infinite guilt.

The blood cannot redden the ocean, but the blood is not blood, it is used here as a metaphor for guilt. The picture of the ocean turning red gives you a violently dramatic notion of the infinite blackness that now burdens Macbeth's soul, something you couldn't get if he had merely said, "Oh, my guilt is infinite."

A literalist who sets about calculating the effect on the ocean of a bloody hand is getting no value out of what he reads.

One more example. Consider Coleridge's "Rime of the Ancient Mariner. " In the fourteenth verse of the third part, there come the lines: "Till clomb above the eastern bar the horned Moon, with one bright star within the nether tip."

The "horned Moon" is the crescent moon, of course, and there ca n't be a bright star within the nether tip. The crescent is the lighted portion of the moon, but the rest of it, though out of the sunlight and dark, is still there. For a bright star to be within the nether tip is to have it shining through hundreds of miles of lunar substance. It is an impossibility, and I don't know how many readers have snickered at Coleridge's naivete in this.

But is it naivete? The poem begins very simply and naturally till the Ancient Mariner kills the albatross, a lovable and unoffending bird. This itself is a metaphor. After all, human beings have killed lovable and unoffending birds since time immemorial. In this case, though, the killing represents all the callous and indifferent cruelty of the human species, and, as a result, the ship with its crew (who approved the Mariner's deed) enters a strange world in which natural law is suspended and chaos is come again as God removes himself. The atmosphere of the poem becomes weird and unearthly and normality begins to return only after the Mariner involuntarily blesses all the living things in the ocean in a gush of love.

I have a feeling that Coleridge knew that a star could not shine within the nether tip of the crescent but merely used it as one more example of the chaos of a world in which human cruelty denies love, order, and God's presence. It is only fitting that a star shine where no star could possibly shine.

To miss that point is to miss the point of the poem and to understand only its jigging meter and its clever rhyming-which is plenty, but far from enough. A literalist deprives himself of the best part of art.

Suppose we apply this way of looking at things to visual art. If you ask an artist to illustrate a piece of writing precisely, you make of him a slave to the literal word. You suppress his creativity and impugn the independence of his mind and ability. The better the artist, the less likely he is (barring an absolute need for money) to accept such a job.

An artist worth his salt does not illustrate the literal words, but the mood of a story. He tries, by virtue of his art and ability, to deepen and reinforce the meaning of a story and the intent of the writer.

Thus, in the mid-December 1988 issue, the cover of Asimov's illustrates my story "Christmas Without Rodney." It does not illustrate any incident in the story. Instead it shows in the foreground a boy with a sullen and self-absorbed expression. What's more, the predominant color is red, which to my way of thinking symbolizes anger (a metaphor for the flushed face of a person in rage). This demonstrates the anger of a spoiled brat who does not instantly have his own way, and the anger he inspires in the narrator of the story. Behind the boy is an elaborate robot, with one metal hand to his cheek as though uncertain as to his course of action, something that fulfills one of the underlying themes of the story. The artist, Gary Freeman, does not illustrate the story, but adds to it and gives it a visual dimension. That is what he is supposed to do and what he is paid to do.

This brings us to the cover illustration of Agent of Byzantium. It is clearly the intent of the artist to illustrate the n ature of the story, not the story itself. Constantinople is in the background, identified by the gilded dome of Hagia Sophia. In the foreground is a soldier who has Byzantine characteristics. So far we have an historical novel. But he also possesses objects of high technology associated with modern western culture. Clearly it is an historical novel set in an alternate reality. And that is what the book deals with. The cover is precise, it tells us what we need to know, it satisfies the artist's own cravings, and if the details of the technology are not precisely met in any incident in the book, that matters not a whit.