HERE ARE five novels that have been labeled fantasies for young readers. One of them is the first such work by a popular writer of adult fantasy, three the work of established children's writers, and one the sequel to a well-known children's book by a new author writing her first book. "Young reader" may prove as capacious a category as "fantasy," but what unites these books is that all really are novels, not extended stories, and all try in different ways to shed the fairy-tale image likely to linger over any book written for "young readers" that is a fantasy besides.

Jane Leslie Conly's Racso and the Rats of NIMH (Harper & Row, $12.50; ages 10-up) takes up the story of Robert C. O'Brien's Mrs. Frisby and the Rats of NIMH a few years later on. Returning us to the field mouse household of the Frisbys, Conly tells how Timothy, Mrs. Frisby's youngest son, undertakes to cross the countryside to attend a school run by the superintelligent rats of NIMH after Jeremy, the voluble crow from the earlier book, proves unable to take him. Along the way Timothy encounters Racso, a brash and rather overbearing city rat whose father had been one of the rats of NIMH, who is also bound for the school. After various adventures the two animals reach their destination, where they learn that a crisis looms: the valley is about to be flooded.

The story of Timothy and Racso moves swiftly and enjoyably, departing very little from the tone or range of the earlier book. Conly does press the book's moral lessons a bit harder than Robert C. O'Brien did, and seems rather conservative in her treatment of the characters he created, like a guest in another's house. Contrasting an author's first book against the very high standard of Mrs. Frisby seems a bit unfair, but is hard to avoid when sequels are written by different authors.

A larger problem is that Racso and the Rats of NIMH appears to be written for a slightly younger audience than its predecessor, which raises the question of who is going to read the book. However, as Mrs. Frisby has engendered both a film and an abridged edition for very young readers based on that film, a readership probably does exists who are familiar with the story. It is too bad that readers who enjoyed O'Brien's book may find this a bit too easygoing. Conly writes intelligently and well, and I hope her next book takes up a story of her own. The Realm of Dreams

ELIZABETH A. LYNN has written both science fiction and fantasy for adults, and The Silver Horse (Bluejay, $7.95 paper; ages 10-up ) marks her first book for younger readers. It is an appealing debut, which charmingly contrasts present-day San Francisco, where the story begins, with such otherwordly places as the Realm of Dreams and the Land of Runaway Toys. Susannah, an 11-year-old who chafes at having to share a room with her younger brother Niall, is plunged into an adventure when a painted wooden horse Niall had been given comes to life. The horse turns out to belong to the Dreamkeeper, empress of the Realm of Dreams, who dropped it during one of her night-time forays to the mortal world. When night falls the horse returns, taking Niall with it, and Susannah must journey through lands of magic to rescue him.

Susannah is very much a present-day girl, who knows she is embarked upon an Adventure such as story-books tell. She and her friend Danielle find some of their expectations contradicted, however, when they encounter such oddities as a vengeful band of mistreated dolls, or a teddy bear who is offended when people assume its name is Teddy. Before venturing into Dreamland, Sussannah and Danielle are warned that the Dreamkeeper will try to lure them into everlasting dreams, and the girls must confront the empress of Dreamland before they are able to return.

The Silver Horse is a swift and graceful book, sure to give pleasure to young female readers. Boys might be less happy with it, however, as Lynn has kept the leading cast resolutely female -- those helping Susannah and Danielle include a beneficent witch, her young and beautiful apprentice, and her familiar (a spider, but given the female pronoun), while the only males, aside from the whiny Niall, prove to be two rather helpless victims of the Dreamkeeper. Talk With the Animals

Alan Arkin's The Clearing (Harper & Row, $12.95; 10-up) has wise things to say about aggression and pride, but has trouble finding the artistic form in which to say them. The Clearing is a sort of spiritual retreat for a group of talking animals who look to a Bear, who has attained a kind of nirvana, for spiritual guidance. Bubber the lemming, who has no purpose in life after refusing to join his community in mass suicide, is brought to the clearing, and joins in the effort to find meaning in life. Like many of the animals following the Bear, he is something of a schlemiel. ''Well, what can I tell you?'' asks a self-loathing snake in one of the novel's funnier scenes, in which Bubber unwisely insists on hearing his life story. ''I ate him. I swallowed him whole. That's what happens, that's what boa constrictors do. Let's face it.''

Despite some affecting pages, The Clearing grows tedious in its second half, for there is little action in the book, and Arkin is not able to sustain his story on the strength of his language. Knights, Kings, and Viziers

CHERRY WILDER has written science fiction for young adults that has the flavor of fantasy, and with The Summer's King (Atheneum, $13.95; ages 10-up), the concluding volume of her trilogy The Rulers of Hylor, she gives us fantasy with the flavor of historical novels. Wilder is writing in the mode of high fantasy here, with all the trappings of knights, kings, and viziers. No dragons, however, for in the land of Hylor magic is difficult and plays little part in the daily lives of commoners or noblemen. Such forebearance allows Wilder to develop a rich texture to her depiction of the reign of Sharn Am Zor, the Summer King, creating a novel of unusual complexity and depth.

Sharn Am Zor, at 22 a king and co-ruler of a prosperous and peaceful land, is impetuous and willful, impatient with onerous necessities and heedless of his brother and sister, whom he grants a lesser place in court than his favorites. Sharn is moved to seek his bride in the distant land of Eildon, where magic holds greater force and royal courtiers must undergo arduous trials. Disregarding counsel, the king embarks upon a disastrous journey to Eildon, where his ambitions founder but a measure of wisdom is finally gained.

So bald a summary suggests little of the book's wealth of detail or preciseness of observation, well equal to that of an adult novel (which is how its two predecessors have been published in paperback). The Summer's King is one of the most demanding books I have seen published as being for young adults, even if one starts at the beginning of the trilogy. It is also among the best instances of "high fantasy" published in many years, a pendant in a genre not known for celebrating maturity or wisdom. The Rulers of Hylor will not be for everyone, but young readers who have enjoyed Cherry Wilder's earlier work will be entranced. The Eldest Sister

AFTER THE high lore of Hylor, Diana Wynne Jones' Howl's Moving Castle (Greenwillow, $10.25; ages 10-up) is a decided return to Earth. One of Jones' chapters is entitled "In Which Sophie Enters Into a Castle and a Bargain," and the last is called "In Which a Contract is Concluded Before Witnesses," and an air of British middle-class common sense prevails throughout, despite the fact that Sophie has entered that castle because a witch has spitefully turned her into a crone.

The eldest of three sisters (which of course guarantees she will be first to fail when the three venture forth to make their fortunes), Sophie must seek help from the fearsome wizard Howl, whose castle moves around and who is rumored to eat the hearts of young girls. What follows when Sophie, impersonating a humble cleaning woman, is turned loose inside the moving castle of Howl -- who proves to be a lady killer only in the figurative sense -- is both inventive and very funny. Jones knows how to fold some good sense into her story without making it too heavy to rise, and Howl's Moving Castle can be enjoyed by readers of almost any age.