ENJOYMENT OF A WORK of fiction does not made it a good work; we have all sometimes liked bad novels and hated good ones. What is sufficient to make a book worthy is that it also be about something substantial, emotionally moving and intellectually stimulating. It follows that many substantial works are not always enjoyable, while many enjoyable works are empty-headed. It would be nice to have both, and it does happen sometimes.

Joseph O'Neill's Land Under England (Overlook Press, $11.95) is a lost race novel, originally published in 1935. The hero, Anthony Julian, descends to a dark central sea in a large cavern under England, in search of his father, a lover of things Roman, who disappeared years before; it seems that over the generations quite a few Julians have disappeared under Hadrian's Wall. The novel is a nearly flawless adventure story, filled with striking images: the Roman galley anchored on the shore of the sunless sea; the Dark City of the Roman state, 2,000 years old; the green glow of the strange lamps and huminous forests; the magnetic aurora in the upper reaches of the cavern; black mountain ranges; Anthony's first meeting with the hawk-faced Roman sea captain.

We are introduced to a totalitarian society which has mastered mind control and telepathy, which out of fear of the darkness and the recognition of human frailty has forged a pyramidal social system based on the need for survival. O'Neill builds an honest case for this society, showing it to us with compassion and deep understanding of its origins before opposing Anthony to its ways; we often feel that Anthony is completely wrong about this world, and that his own convictions are fatuous.

In the end, though, a balance is struck, and we are with him as he confronts his vastly changed father. The final chase through the dark lands, as Anthony tries to regain the realm of light, is deeply symbolic and beautiful; his relationship with his father lends an Orpheus-like undercurrent to the story. The stresses and strains of the finale, as Anthony discovers that his robot-like father is tracking him this father dreams of leading Romans up to reconquer the earth), bring us up against the growing fascist shadow of the 1930s, and face to face with ourselves. Except for a slight wordiness halfway through (the story is told in the first person), this is a genuine classic, enjoyable and profound, and about much more than its own vivid inventions.

Frederik Pohl's The Cool War (Del Rey/Ballantine, $10.95) refers to a kind of low-level international conflict, some 40 years from today, involving political, economic and social sabotage by teams of workaday spies. The Team recruits one. Hornswell Hake, a 39-year-old Unitarian minister, and sends him to infect Europe with a virus. After this rite of passage, Hake learns that his success is due to his being manipulated, physically and psychologically, as much as to his own actions. His growing revolt against human nature and his employers takes place against a minutely detailed 21st-century world. Much of the story is a droll James Bond mixture of technology and humor, leavened by Pohl's clear reservations about human nature: Where will it all end? Hake wonders as his actions grow more byzantine, and human evil begins to seem like the most natural thing in the universe. The story reminds me of vintage Heinlein, with its savvy about how things actually work; there is a bit of sentimentality once in a while, but it's of the tough "you have to do the best you know how" kind.

The book is a pleasant, crackerjack read, with serious overtones and striking backgrounds belonging to a vividly realized future. Gradually, a sly sense of hope about people emerges; our hero takes his stand and wins a limited moral victory. An intelligent adventure, the book occasionally threatens to become more, but Pohl holds back, almost as if he were embarrassed.

Best moments: a field of flowers turns the sun's rays toward a giant solar collector which stands like a giant spider off the Arabian coast. Crisp, satirical, ironic; Graham Greene, if he had written this novel, would have called it one of his entertainments.

The Dreamers (Simon and Schuster, $10.95) by James Gunn is especially relevant to the writing of science fiction, since it deals with the relationship to reality. Vicarious lives are created for a future humanity through the use of computers and the chemical transfer of memory. Humanity lives in huge urban centers run by single mnemonists, who channel information through their minds.

The story turns on the search for a replacement mnemonist, an original mind to care for the social organism, lest it collapse. The search reveals to us the lives of a doctor, an historian, and a dreamer. The dreamer is a kind of future fiction maker, who creates lives for others to live. This future is presented through a number of typeogaphical tricks, juxtaposing different kinds of information to suggest the way that the mnemonist would see things.

In this novel about wallowing in dreams, Gunn takes a pretty sad view of human nature, which is seen following the line of least resistance and choosing a dream life over the real. The book suggests a clear distinction between dreams and reality: real events are irreversible, but dreams can always be changed. Gunn thus puts his finger on the core of meaning -- playing for keeps in the dream that cannot be reversed, namely reality.

In one sense the novel argues with other science fictional works, since it suggests, by implication, that meaningful creations must carry a strong reference to matters outside of themselves (in the same way that a scientific theory must rejoin experience through an experiment); the alternative is formal and personal solipsism, which is a kind of death, since meaning (playing for keeps) is lost.

Gunn's language is always sure, efficient and evocative (the switching of tenses between the waking and dream states is especially effective), building to a memorable last line. It has been pleasant and exciting over the years to watch Gunn go on producing work that is full of personal integrity, innovation (the book fuses a variety of techniques into a seamless narrative) and imaginative strength, while lesser lights have come and gone, carrying away undeserved prizes.

While The Dreamers incisively explores the nature of dreaming, Windhaven (Timescape/Simon and Schuster, $13.95) by George R. R. Martin and Lisa Tuttle presents us with a dream that is only about itself. Flyers connect the islands of a low gravity ocean planet, where a star ship once crashed. The survivors fashioned wings from the vessel's interstellar sails (metals are scarce on this world), and these are passed down through the bloodlines of flyers; wings are becoming scarcer, since not every inheritor is a good flyer. Maris, the heroine whose life story this is, leads a successful revolt against tradition and succeeds in founding academies of merit, this follows the novel's first crisis, when Marie almost loses her wings to her foster brother. Her success, however, is only the beginning of her problems, since her changes produce further instabilities in the culture.

It is to the authors' credit that they show things getting tougher and more complex as Maris grows older. Unfortunately, the book's sentimentality is as unyielding as its melodrama and its reaching after easy truths. The posturings of the characters too often seem gratuitously false, and in the end the obvious artifice and convenience of the whole conception is overwhelmingly tedious, nothing can redeem the ooh-aah, gratuitous wonder of the exercise: let's have a planet where we can flap around and have brooding fun. Despite quite a few vivid moments, the narrative suffers from the weight of its own rhetoric and the frequent tears of its characters.

Windhaven gives the impression of bits and pieces from everywhere, all woven into a derivative tapestry. The writers, a skilled and intelligent pair, are the victims of great applause. It is human to go where you are wanted; it's unthinkable to them that they are doing anything questionable. I wanted to like this book, and I did enjoy it in some ways, but that is far from saying that it's good. Why? The novel is about itself, mostly, and where it touches on themes outside of itself, they are presented in a hackneyed, almost mock-dramatic way. Such sentimentality obscures the hard truths that an author must tell about his characters and their world, in order to make the hardness bearable, but the task of art is understanding and illumination. The furious action and tearful, romantic pseudo-catharsis of this story reveal only a stage set. There seems to have been no need to treat its themes of childhood, role models, and the reforming of tradition as a science fiction story; science fiction belongs to material that cannot be presented better in any other way. A practiced piece of kitsch, this will be a very popular book.