CULT BOOKS come and cult books go -- that's part of what it means to be a cult book. A few keep reappearing, however. They get discovered over and over by successive waves of admirers. After the third or fourth reappearance, the suspicion begins to arise that this isn't a cult book, after all. It's a masterpiece with problems.

Islandia is such a book. This obscure novel, the lifework of a man who wasn't even a professional writer, has had a devoted following for 40 years. It has gone out of print numerous times and always triumphantly returned. Some of its admirers would emigrate in a minute to the country where the story takes place, if only they could find it. Others know the book very nearly by heart, even though it's a thousand pages long. All this s "cult."

That Islandia is also a masterpiece is what I'm about to argue. And that it has problems, no one who learns the plot is likely to dispute.

Islandia began as the fantasy world of a small boy named Austin Tappan Wright, around the year 1890. In the beginning it was a fairly commonplace fantasy world. The little boy imagined an island kingdom. There people rode horses from castle to castle, instead of taking trains from city to city, as his actual family did. (His actual family was quite high-powered and very modern. The father was dean of the Harvard graduate school, the mother a novelist.)

Many children have such fantasy worlds, and most abandon them at the beginning of adolescence. Wright did not. He grew up to be a distinguished professor of law, a husband, a father, all of that -- but he continued to live part-time in Islandia. (You pronounce it Iz-landia. All consonants in the Islandian language are sounded.)

As he grew, so did his imaginary world. It ceased to be an island and became the southernmost part of the Karain continent, somewhere below the equator. It kept its king, but he became very much a constitutional monarch, not an absolute ruler. It dropped the castles. It kept the horses -- but they were no longer part of the romantic imaginings of a small boy. They were part of the utopian vision of a fully mature man, one who has thought out the relationship between speed of travel and meaning of journey. It gathered a two-thousand-year history, a complete sociology, a stunning cast of characters, a theory of how human beings can best lead satisfying lives. In the end, Islandia became the greatest vision I know of a life in which high culture coexists with low technology.

The story the book tells is vastly too complicated to summarize here. I can give some indication of how it begins, though. It begins with a young Harvard student, John Lang. In his freshman year he meets an extraordinary and charismatic classmate named Dorn, said to be from the hermit kingdom of Islandia. (Islandia expelled Saracen invaders in the 12th century, Christian missionaries in the 16th, and, ever since a British attempt to establish treaty ports in the 1841, it has had a law permitting only 100 foreigners in the country at any one time.)

Dorn and Lang become friends, and the American winds up learning a great deal about Islandia: its farm economy, its small size (the population is just over 2.2 million), its unusual theories about the relation between mind and body. Not long after the two graduate, there is a fresh attempt by the great powers to compel Islandia to enter the modern world as defined by them. Under gunboat pressure, Islandia agrees to accept diplomatic representatives from the major countries on a temporary basis. Partly because he knows more about Islandia than most Americans, and partly because he has a rich and influential uncle who means to use him as a cat's paw in getting trade concessions, Lang becomes the first U.S. consul in Islandia.

TWO PLOTS now move forward simultaneously. One tells the story of Islandia itself over the next two years. That tiny country has to decide whether to yield itself up for development, or even at the risk of destruction to keep its own way of life. This is a thrilling story, thrillingly told. People who advocate development will find it so just as much as those who don't. Among its other virtues, Islandia is a page-turner.

The second plot is John Lang's personal story. Here the book is mainly about love. Lang is 24 when he becomes U.S. consul, and he is emotionally an intense person. During the course of the book he has three great romances, two with Islandian girls and one with an American. This last begins when he is back home for a time, having lost his diplomatic post. (He failed to please his uncle and the other powerful businessmen who wanted the Islandian coal and copper.)

Islandians have very different views about love than Americans do. So different that they have to use three terms instead of our one. Love takes the forms of alia, ania, and apia. Partly because it's hard to do it out of context, and partly in the hope of making you so curious you'll read the book, I shall define none of the three -- except to say that apia has a diminutive form, apiata, and that corresponds somewhat to the English word "lust," though lacking the pejorative connotation.

Few people have been better than Austin Wright at describing the nuances of the various emotions we lump together as "love," and in this way, too, the book is a masterpiece.

But there is still one plot element I haven't mentioned. It is this one that makes Islandia a masterpiece with problems. The kingdom of Islandia faces other threats besides those of European colonization and American exploitation. There are also wild tribesmen who live across the mountains, right there in the Karain continent. They're called the Bants, they've recently been armed by the Germans (who have established a protectorate in part of northern Karain), and they love to come over the mountains and raid Islandia.

No harm so far: That just makes for more excitement. The problem is that, while the Islandians are white, civilized and very appealing, the Bants are black, uncivilized and fairly awful. It makes a reader uncomfortable.

And yet -- this is a book I have taught to college classes that included black students. They were not thrilled at the role played by the Bants, but some of them liked Islandia itself very much. All, I think, conceded that Wright's focus was not on racial prejudice. He wanted a danger over the mountains. In the end it wouldn't change the book much if it were black Islandians threatened by white Bants. Furthermore, race in the genetic sense is simply not his concern. You learn in the course of the book that the Islandians were once scattered in tiny groups all over the Karain, and only coalesced into a nation after the Bants arrived. Written records do not go back that far, but present-day Islandians take for granted that their ancestors include some of the early invading Bants. This is a world that Austin Wright invented; and if he had wanted racial purity, he could easily have had it. Obviously he didn't.

But it's not for the Bant wars that one would read Islandia in any case. One would read it partly for the great vision of what life would be like if there were no rat-race and no fast lane. And one would read it partly for the characters. There are about a dozen who are not only three-dimensional, but damn near four-. This gives them 21/2 more dimensions than characters in utopian novels generally have. To us cultists these characters are unforgettable. I think they would be to anybody.

Note on availability: "Islandia" is currently out of print. Copies both in paper and hardback are fairly easy to find, though. If you can, get either the original 1942 hardcover, published by Farrar and Rinehart, or the 1958 reprint. Both are exceptionally handsome books.