In Frederick Pohl's "Beyond The Blue Event Horizon" (a sequel to his "Gateway," which took practically every science-fiction award available), the time is a not-very-distant future, human technology is recognizably related to that which we now enjoy and suffer, the world is beset by an enormous population problem, heading for a food crisis and rapidly being stripped of its remaining resources to stave off emergency.

The chief difference from the world of 1980 is an enormous one: the looming shadow of the Heechee, an alien species who have never been seen by humans, but whose discarded artifacts are scattered across the universe to puzzle and tantalize human explorers and occassionally, when some fragment of their mysterious technology can be deciphered, to make the discoverer rich and famous.

This background was richly presented in "Gateway," a novel whose ideas were too good -- too complex and steeped in mystery -- to be exhausted in a single book. Its sequel has been awaited for more than two years -- more eagerly, perhaps, than any in science fiction except the conclusion of Philip Jose Farmer's "Riverworld" series -- but not without some trepidation. Sequels are often a serious problem for novelists, in science fiction as in the mainstream.

The question of the prayer fans, as it happens, is the central one of the book; once it becomes clear that these strange artifacts, which have been collected as exotic art objects, are actually stores of information (including such earth-classics as "Robinson Crusoe"), Earth has access, suddenly, to a vastly superior technology and its problems are theoretically solved. But this breakthrough is embedded in an intricate plot involving romantic, scientific, economic and life-threating ups and downs for a large cast of human characters and one non-human species, as well as some very likeable computer programs.

There was no need to worry; pohl is one of the most expert technicians in the field, and he carefully exploits the chief advantage of a sequel while avoiding its chief drawback. The advantage is particularly strong for a science-fiction writer, whose major technical challenge is usually that of sketching in the background of the book (which is frequently more important than any of the characters) without letting the story get bogged down in explanations of the strange, new world he is creating. In his new novel, Pohl is able to do this simply by incorporating "Getaway" as the background for "Beyond the Blue Event Horizon."

This means that a reader who tackles the second book without having read the first may suffer some disorientation on a first encounter with, for example, a "Heechee prayer fan" -- which turns out, in this book, to be unrelated to prayer and to be an information-storage system, in effect a book, rather than a fan. But most devotees of science fiction will have read "Gateway" already, and the few who have not will find enough clues in the book's context of keep them going.

The chief drawback of sequels is the reader's sence of deja vu, the feeling that the second book's materials are slightly shopworn. Pohl solves this problem by writing a completely different kind of book the second time around. "Gateway" was a prime exemplar of what might be called the "brass tacks" school of science fiction. A cast of characters recognizably like people we know spent the novel coming to terms with hard economic facts and baffling technological mysteries in an environment (the Gateway station, chief base for the launching of explorations of distant space) which nobody has yet seen but which can readily be imagined and is meticulously described. As can be judged from the title, Gateway is the dominant character of the novel.

In the sequel, the key word of the title is "Beyond," and this fact characterizes it as a sample of what might be called gee-whiz school of science fiction. In a nutshell, Pohl spends most of the first book taking an imaginative new look at technology, most of the second cultivating the reader's sense of wonder.

There is a danger that the distinction between these two kinds of writing may be lost from sight, because in the last few years science fiction has become one of the hot areas of movie-making, and in science-fiction movies, the gee-whiz element usually focuses on technology: The audience sits transfixed at the spectacle of a giant spaceship moving out of drydock or battling in deep space, and the eye is so dazzled that the mind can easlily ignore the fact that such magnificent visual spactacles have no significant fictional or intellectual content. One stares at a brass tack in wonder, and this is a splendid experience, but it has little to do with the best science fiction, which is a feast for the imagination and the intelligence rather than the senses.

In contrast, Pohl's work offers science fiction at its best: riddles posed, examined and solved; basic human problems -- guilt, hunger, adolescent insecurity and older people's loneliness, revenge as a driving force, understanding as a counterforce and mystery as an inevitable fact of life -- woven deftly into an intricate plot; pure adventure happening to believable (if not deeply drawn) characters in surroundings almost beyond the borders of imagination; and at the end, when other questions have been laid to rest, the posing of a new question is unfathomable as time and space themselves.

It is, like all good science fiction, a mind-expanding experience.