THE SERIES has always been prevalent in science fiction, and with perhaps more justification than in other genre. Publishers in any field like series because they are a sure thing -- success of the first book guarantees a certain number of sales for subsequent volumes. For the reader, too, there is a certain safety in the series, a cosy absence of surprise.

For the writer, the serfies can be a comfortable trap, a way of avoiding something more challenging. Or it can be a way of exploring a compelling inner landscape, a way of establishing rules and exploring the possibilities and consequences of them in more detail than one novel might allow. Having gone to all the trouble of creating a convincingly alien society or a detailed prediction of the future, the science fiction or fantasy writer may be understandably reluctant to abandon it after only one use. There are as they say, a million stories in the naked city -- not to mention Darkover, Dune or Mars. It's not only the reader who longs to keep exploring an imaginary world after the book ends.

Elizabeth A. Lynn's The Northern Girl (Berkley, $13.95) is the final volume of a trilogy called The Chronicles of Tornor, but it is complete in itself. The three books share neither story nor characters, but a common background and culture presented in different stages of development and decline.

The land of Arun is an impressive creation. Not because it is so alien (it is not; the origins of most of the exoticism like the concept of cheari, the importance of dance, the ruling witches, etc. are familiar and obvious ones) but because Lynn makes it so real and accessible. Her language is clear and simple, always elegant, and her sentences are rich with sensory detail. From the very first page, as the heat, the still air, the dockside smells and sounds of Kendra-on-the-Delta are evoked, Lynn's world springs vividly to life.

Arun might have existed in our distant, forgotten past, or in an equally distant post-technological future, or only in an alternate reality. It's a very appealing place. The people there seem happier and more in tune with themselves, nature and each other than most of us are. As in the fiction of Vonda McIntyre, the society presented here is totally, and convincingly, free of all the traps and scars of sexism. It is possible to read The Northern Girl and really understand what it would mean to live in a world without crippling sexual stereotyping or repression. This is no utopia, but a human, believable world where people lie and hate and die as well as love and grow, and where the villains are as comprehensible as the heroes.

The actual storyline doesn't quite live up to the richness of the background, despite such classic elements as the corrupting effects of power and envy, and an adolescent seeking to come to terms with her heritage. Depsite much talk and movement, the plot when fully revealed is disappointingly slight. The details of it are already fading in my memory. What remains are the land, the language and the people.

By contrast with The Northern Girl, Marta Randall's Dangerous Games (Pocket Books, $2.95) comes off looking rather flat and uninspired, although it is an enjoyable, well-plotted adventure. It's the second in a series (Journey was the first) about the empire-building Kennerin clan, a family saga in outer space.

The story takes place on board spaceships and on several different worlds, all with the air of Hollywood backlot sets about them and all nearly interchangeable. The sense of place which is such an integral part of Lynn's book is sadly lacking in Randall's. Even the Kennerin's own planet, Aerie, is only dimly realized. Aerie is a world of islands, and yet there is no sense of the presence of the sea, no feeling for the particularities of island life, no clear picture of individuals acting on and being influenced by their surroundings. Randall's language is competent and utilitarian. It draws no attention to itself and becomes a part of the bland background.

But the characters rise above these flaws to command the reader's attention.

Randall has created some memorable and fully-realized characters, and she is particularly good at conveying they way people interact with each other, the unspoken expectations and fears of lovers, family, friends and strangers. Perhaps because it is a part of a series, there is an attractive complexity to many of the characters, a sense that what the author is showing us is only a piece of a large and interesting whole. But Dangerous Games is over-populated. It gains something from being part of a series, but it loses something -- a sharpness of focus. I grew impatient, at times with the course of the book -- it took so many less-interesting side-trips to account for the doings of everyone. In particular, I could have done without the Kasirene. Randall is skilled at human characterization, but her Aerie indigenes come across as rather dull but peaceful Amerindians stuffed into four-armed kangaroo suits, and they have about them the strong odor of Plot Device.

The Hitchhiker's Guide to the Galaxy, by Douglas Adams (Harmony Books, $6.95) began life as a radio series in Great Britain (National Public Radio will begin broadcasting it here in January), became a stage play and then a novel, and is now in production as a BBC television series.

It's science fiction and it's extremely funny -- a rare and precious conjunction in a field where what usually passes for humor is a bad pun at the end of a dull story.

There's nothing dull about the Guide, which is inspired lunacy that leaves hardly a science fictional cliche alive. It relates, in an almost linear fashion, the adventures of an interstellar hitchhiker who calls himself Ford Prefect (when he's visiting that mostly harmless planet called Earth, at any rate) and his dazed companion Arthur Dent, who is fated to see not only his house but his entire world demolished to make way for a new expressway. On their travels they encounter, among other things, aliens, computers, a depressed robot, the third worst poetry in the universe, and even the long sought after answer to the great question of Life, The Universe and Everything. It's all over much too soon. But -- don't panic -- there's a sequel on the way.

Out There Where the Big Ships Go, by Richard Cowper (Pocket Books, $2.50) is not a series; it's a collection of unrelated, longish short stories set in the distant future, the near future, the present and the past. The weakest of the lot is "Paradise Beach," a bright and fluffy murder mystery; the other four stories are finely-crafted, absorbing tales, alike only in that all are well-written, multilayered, and well worth your time.