A Summer of Transformations : SHELTER: <i> By Jayne Anne Phillips (Houghton Mifflin/Seymour Lawrence: $21.95; 279 pp.)</i>

“The forest is all around us and we’re like a country inside it,” Alma Swenson writes home from her Girl Guides summer in the West Virginia mountains. Her older sister, Lenny, tells herself: “Nothing from home belonged here; home would take it all away.”

Camp Shelter is, among other things, a camp. It has ramshackle wooden cabins and canvas tents, morning river-fogs and stupefying noonday heat, mosquitoes and no-see-ums, campfires, hikes, swimming in a muddy pond, lectures from the director about the Communist threat and the importance of table conversation, and reassuringly glutinous meals. It is a place of homesickness and emancipation from home; of docile little girls with nighttime fears, and itchy adolescents whose fears are indistinguishable from their inchoate desires.

Jayne Anne Phillips’s powerful novel, set in the early 1960s, gets these things absolutely right, in a prose so pungent and particular that a reader feels the heat, fogs and insect-bites, as well as a summer camp’s mix of sensory fullness and lonely voids. It wins our presence--it all but makes us a vulnerable 12 years old--and it needs to, because it will ask a great deal more.

Essentially, Phillips’s theme is the transition between childhood and adolescence. She writes it as a legendary quest; a passage of exploits through dragons, demons and dangerous enchantments, both within and without. Camp is J.R.R. Tolkien’s Middle Earth: the place of questing. It is where children, like Arthurian knights, leave the protection and constraint of what is crowded and familiar to embark on what is solitary and strange.

The questing children in “Shelter,” who range from 12 to 16, are beset by out-sized forces in the persons of two deranged men; one a devil and the other a kind of angel. The children’s involvement in the pair’s deadly struggle wins for them a quest’s transformation. It is a somber ordeal but--unlike the muffled traumas of childhood--an active one. By the end of the book they will begin to be free, in different ways, from their subjection to the dreams and nightmares they have brought, dangerously bottled up, from their homes.

“Shelter” moves, sometimes bewilderingly, between a lyrically perceptive psychological realism and a gothic primal savagery. It is set out as individual voices that converge. There are Lenny and her best friend, Cap, seniors in their mid-teens and lodged in a tent at the top of the hill. The camp is a visible progression: the older the campers, the higher up they are, the farthest from camp control and protection, and the greater the rigors they put up with. Here as elsewhere, Phillips sets up a powerful symbolism that works so well that we sense but barely see it.

Lenny, the central figure and the one who will be at greatest risk, is quick, nervy and eager--still unconsciously--for life. She will get a preliminary taste of it in a midnight encounter at the pond with a young lifeguard, an erotic passage that is both encouraged and cut short by Cap, who uses Lenny as a surrogate for her own hesitant urges. The relationship between the two girls, a partly sensual comradeship before the mysteries that lie ahead, is drawn subtly and fiercely.

The other pair of campers is Alma, Lenny’s younger sister, and her best friend, Delia. At 12, a girl is still easily a hero--a general, a president, a great medical pioneer--and Alma has a heroic mission. It is to protect Delia, heavily traumatized by the recent drowning of her father. The death is ambiguous--he drove off a bridge--and it leads back to the shadows that all four girls will, in the course of the book, struggle away from.

Cap bears the dry solitude of a rich child whose parents live apart and in mutual contempt. Alma’s and Lenny’s mother was having an affair with Delia’s father, and she used and abused Alma as her confidante. Lenny is shadowed by ambiguous images of a different kind of abuse by her father. The ambiguity, though it is a refusal to simplify and though it contributes to the hallucinatory force of Lenny’s quest, has its drawbacks. An author doing something as difficult as Phillips does, and mostly brilliantly, balances depth against movement. The deep folds of Lenny’s consciousness lead to moments of stasis--dispelled in the book’s choking climax--and puzzlement for the reader.

Into the camp setting, so poignantly rendered on its underside and so beautifully drawn in its lively externals--Mrs. Thompson-Warner, camp director and vigilant sentinel against communism and low manners, is the cartoon bubble that an intense book needs for oxygen relief--the dragons enter. They exude the flames of the darkest and most contorted characters of William Faulkner or Flannery O’Connor.

One is Carmody, a violent, vacant degenerate who has come out of jail to prey upon his sturdy wife, the camp cook. She takes his abuse as if it were simply another cross; she is a woman both religious and open, with a bent for lightness and the provision of comfort. For the campers this translates as prodigiously comforting meals; for her elfin 10-year-old, Buddy, it is an instilled openness to the magical possibilities of the woods and fields he roams, Puck-like. A tutelary figure, whose significance grows steadily, he is untouched by the loathsome sexual acts his father requires him to perform. He may, however, perish from them.

But a primal drama requires an angel as well as a devil. It is risky enough for Phillips to introduce such a drama into the lives of her campers, and astonishing that she succeeds in getting it to work convincingly upon them. Even riskier is the angelic figure she chooses.

Parson, a former cell-mate of Carmody’s who follows him to Camp Shelter, is a man possessed. He sees ghosts, preaches visions, does snake magic and fights titanic battles against the Devil and those he believes the Devil owns. His internal monologues are violent memories and mystical diatribes and, as he spies on the campers, Phillips leads us to believe he is another monster. When he encounters Lenny alone, and we hear of her lying motionless on the ground afterward, we are certain it is another horror. Instead--Phillips’s purposeful but difficult ambiguity is in play--it is some kind of redemption.

In fact, in a mode that blurs the hallucinatory with the supernatural, Parson has come to prevent horrors. He will end up saving Buddy from Carmody and empowering the campers to fight Carmody for Lenny. It would not be right to reveal the details of the battles; they manage to be both mythical and endearingly natural. After the final battle, Lenny, Cap, Alma and Delia and the other campers who help them, will be transformed by what they have been through.

Transformation, for Phillips, is the terror, magic and ordeal of what happens year by year as we grow out of childhood. She has set her remarkable novel at the mysterious crossroads where old safety, with its unexplained shadows, becomes more lethal than new danger, with its fearsome ventures.