Spaceships and robots; :The Known and the Unknown: The Iconography of Science Fiction, by Gary K. Wolfe. Kent, Ohio: The Kent State University Press. $12.50

Most readers of science fiction turn to it for entertainment. But unless they are wholly uncritical, they undoubtedly have difficulty sorting out the worthwile from the wasteland of pulp.

Now comes "The Known and the Unknown" to the rescue, offering some explanations of all those spaceships, cities, wastelands, robots, and monsters.

Until recently, most commentary on science fiction reflected the uncritical enthusiasm of the fan or the cataloging of the chronicler. But the growth of the Science Fiction Research Association, a branch of the Modern Language Association devoting itself to speculative fiction, has brought about in recent years an increasing number of critical studies which have done much to loosen the loops of entangled thought of many science-fiction writers.

Gary Wolfe describes his study as "an attempt to explore how a few images familiar to any science-fiction reader . . . have developed into 'icons,' and how these icons are used within specific works, within the genre as a whole, and to some extent within the culture the genre relects."

Defining his icons as cultural phenomena, Wolfe devotes a chapter each to the spaceship, the city, the wasteland, the robot, and the monster. In addition, he discusses in perhaps the most significant chapter of the book the "image of the barrier," since barriers, as that which stands between the known and the unknown , pervade the book.

Wolfe's book is both panoramic and specific, containing not only criticism but extensive plot summary, thus providing the reader with clues for further reading. He also explores in depth some unfruitful novels.

At times, however, one wishes Wolfe had been more judgmental, less like those enthusiastic science-fiction scholars who occasionally lose their sense of proportion -- and humor -- and fail to detect the point at which the wildly speculative tilts over into the ridiculous. For example, there is Wolfe's serious discussion of the problems of Cordwainer Smith's "scanners" -- humans who are modified with mechanical parts to make the pain of space travel bearable. Fortunately, a scientist discovers that scanners are unnecessary if one lines his spacecraft with oysters . . . and, well, isn't this at least faintly funny?

Nonetheless, Wolfe's book is useful for its comprehensive study of this popular and vibrant literary genre. The flamboyance of science fiction, with its tangles of unkempt and monstrous speculations, needs the ordering capacities of the scholar. But its criticism could use some of the verbal rocketry of its subject -- or at least an occasional bloom of gratuitous wit in its concrete city blocks. Let's hope that scholars like Wolfe can surmount the barrier of academic style and move further into the largely unexplored realm of graceful prose as well.