YOU CAN BLAME A LOT on young adult science fic-

tion. I would guess that most adults who now read science fiction got hooked by reading young adult books first. In my case, it was the Robert A. Heinlein "juveniles" and the "Tom Corbett, Space Cadet" series. Today, people can get hooked on young adult fantasies, too, like Ursula K. Le Guin's Earthsea Trilogy. Science fiction or fantasy, they're all symptoms of the same disease, a certain excitement of the imagination, and many readers never recover.

In addition, a lot of young adult novels continue to be read by older readers. Adults regularly read the Heinlein, Le Guin, and other "juveniles," sometimes to reexperience past pleasures they had, but also to discover levels and nuances within the books that young readers might not catch.

The Spanish Smile, by Scott O'Dell, is the kind of young adult book that really excites the imagination. O'Dell has written a great many young adult novels, including the Newbery Award-winning Island of the Blue Dolphins. Smile contains only one pure fantasy element, but it's a great one; an island, just off the coast of California, run almost as an independent kingdom by Don Enrique, who lives there with his daughter, Lucinda. There is no electricity, TV or radio on the island, and Lucinda is only allowed to read books written before the 20th century. The book is told from Lucinda's point of view, and she sees everything colored by the books she has read, one minute looking at romance as she might look at Heathcliff in Wuthering Heights the next minute interpreting her father's actions with a horror straight out of Edgar Allan Poe.

And O'Dell's book gives Lucinda a lot to look at. The Spanish Smile is filled with mad plots to take over California, giant bushmasters (the largest poisonous snakes in the world), huge mausoleums filled with ornate coffins, and hastily constructed walls that may hide the remains of one or more bodies. And, because all these fantastic things are told from Lucinda's 19th-century romantic point of view, O'Dell not only gets them to work, he turns the novel into a headlong romp, something I couldn't put down as I waited for the next wonderfully unbelievable thing to happen.

If I had started out reading books like The Bell Tree, by H.M. Hoover, I might not have gotten hooked on fantasy and sf after all. It's not that The Bell Tree is all that bad. Hoover has written a number of other young adult books, and she can tell a reasonable story from a teen-age girl's point of view. Fifteen-year-old Jenny goes on vacation with her father to the planet Tanin, a newly colonized world that humans are adapting to their own needs without much thought to the existing ecology. Jenny's father is an important industrialist who makes a hobby of archeology, and the two of them, along with their guide, Eli, explore the remains of an ancient civilization that they discover in a hidden valley.

It's in these later sequences that the book really falls apart. All sorts of strange things happen which seem to be controlled by the planet's original inhabitants, although whether those inhabitants are still around, or they've just left some sort of warning system behind, is never explained. For that matter, hardly anything is explained or sufficiently explored, including the consequences of the main characters' actions. And, besides a slowly growing friendship between Jenny and Eli, the characters don't change much either. Hoover has brought a certain amount of imagination to The Bell Tree, but she seems unable to use it to any great effect.

Lawrence Yep fares somewhat better in Dragon of the Lost Sea, a fantasy loosely based on a Chinese fairy tale. A dragon, Shimmer, and a homeless boy, Thorn, form an uneasy alliance in pursuit of the witch, Civet, who has stolen the inland sea the dragon once called home, imprisoning the sea in a pebble that hangs from the witch's neck. As the story progresses, friendship grows between boy and dragon, cementing the alliance. Along the way, Yep makes some gentle fun of the prideful attitude of Shimmer, who, after all, is a princess among the dragons, and the even more boastful Monkey, a magical creature who refers to himself as "the Great Sage Equal to Heaven," but never seems to quite finish what he sets out to accomplish. In all, Yep's book is enjoyable, if unexceptional.

The Kestrel, by Lloyd Alexander, like the O'Dell is based on a single element of fantasy. The novel is set in an imaginary kingdom, Westmark, which seems to be located in a place much like Europe around the close of the 18th century. Everything else in the book is quite realistic, although a few of the characters, especially the Count Los Bombas and Musket, his midget sidekick, are a bit fanciful.

The Kestrel is the sequel to Westmark, and Alexander spends the first few chapters reintroducing us to characters and situations from the earlier book. Alexander is a master craftsman, with a great many other books to his credit, including the Newbery Award-winning The High King. Once he gets the action going, he never lets it stop. The Kestrel concerns an invasion of Westmark by a neighboring kingdom, and how it effects Mickle, the young queen of Westmark, and Theo, whom Mickle plans to marry, as well as a number of lesser characters. Alexander gets you immediately involved with Mickle and the others, but it is his portrayal of Theo, who turns during the course of the war fromman idealistic youngster into the hardened leader of a band of guerrillas (or "irregulars," as the books calls them), that really carries the book and gives it its power. The setting for The Kestrel may come from Alexander's imagination, but the pain Theo feels is real, and the change that overcomes him could effect any of us. It is this characterization that makes The Kestrel a novel that can be appreciated not only by young adults, but even by some of us older adults, too. CAPTION: Jacket Illustration, copyright (c) by Charles Mikolaycak from ''The Kestrel''