JULIE'S DAUGHTER. By Colby Rodowsky. Farrar Straus Giroux. 231 pp. $11.95. Ages 10-16.

WITH SEX, death, drugs and the other assorted ambiguities of real life no longer considered too mature for young adults, the final literary frontier may be one of style. In her new book Julie's Daughter, Colby Rodowsky makes an assumption that would surely flatter any intelligent teen-ager: If sophisticated, subtle emotions are fair game, then why not sophisticated, subtle language?

Rodowsky tells her story in three continuing monologues by three people: A 17-year- old girl named Slug October, whose teen-age mother abandoned her as a baby; her mother Julie, who slept with so many men she does not know who Slug's father is and has lived alone for the last 17 years; and Julie's neighbor Harper Tegges, an aging artist dying of a brain tumor whom Julie and Slug care for in the last days of her life.

Slug, raised by her grandmother and only reunited with her mother Julie at her grandmother's funeral, has for years collected "Julie-facts," but the reality is much less comforting than the scattered knowledge she once subsisted on. The catalyst that brings Slug and Julie together is Harper, an independent, creative, feisty painter.

"They brought me home," Harper says after she has left the hospital, "and now my house has been overrun. By a plague of locusts. They cluster there, hanging on to tree trunks and in the bushes. In my studio, dangling from the walls and window frames. Rubbing their legs together. Making noise.

"Just until. Until what? Until I'm on my feet again. Can do for myself. Can work: paint. Because work is all. Until. Until. Oh, Christ. Until I die.

"But I won't. Not yet. I'll fool them. Make a deal . . . Maybe Mama'll come. And Aunt Agnes -- she was the caretaker of the family. The one who came with her tisanes and mustard plasters.

"Richard. Suzanne. She can't, can she? She doesn't know who I am, I don't think. Did Richard ever tell --

"I'm muddled now. Things are muddled and there's a pain in my head. I have to go slow."

Harper's memories, fears, satisfactions and guilty frustrations unfold through a kind of pared-down stream of consciousness. Richard and Suzanne, it is soon clear, are Harper's abandoned husband and daughter, but Rodowsky allows the dying woman's thoughts to wander from name to name, time to time, and never intrudes with excessive explanation. Slug and Julie speak in more conventional voices and their chapters lack some of the resonance of Harper's, but because of the subtlety and humor of the portrait Rodowsky paints of the artist, the affection Slug and Julie feel for the woman -- and their resulting reconciliation -- is completely understandable.

All three are prickly characters, proud, selfish at times. Rodowsky surrounds them with equally strong peripheral figures, like the saccharine neighbor known as "Mattie- Miller" who pins pictures of cute animals on Harper's wall in a futile attempt to cheer up an artist more interested in unsentimental abstractions than cuddly puppies. When Slug begins a romance with David, a young artist also caring for Harper, Rodowsky employs her characteristic restraint. The self-contained Slug would never think in exclamation points, and Rodowsky does not impose any on her.

As Harper's mind and body fade, her voice fades from the book, and the reader feels the loss. Slug and Julie are left alone together. Daughter has begun to understand and even to forgive her mother, and the future promises well for them. But it is in Harper Tegges that Rodowsky has created a character and a voice that makes Julie's Daughter a remarkable book.