Live Forward, Understand Backward : FIRST LIGHT<i> by Charles Baxter (Viking: $17.95; 286 pp.) </i>

“First Light,” intent on its gently resonant, subtly compounding complications, unfolds in a queer way: backward. Charles Baxter’s serenely unflashy first novel comes dressed with a promissory epigraph from Kierkegaard: “Life can only be understood backwards; but it must be lived forwards.” In a kind of reverse Bildungsroman, then, “First Light” begins with adult characters and gradually, chapter by chapter, setting up a rich system of echoes and connections, reduces them to children. “Reduces” is probably the wrong word: The reader experiences an avid interest in these children, having known them as grown-ups. There is the kind of interest an elderly woman acquaintance demonstrates when she claims, “I knew you when you were a child, “ thus declaring her special stake in you, and yours in her--she knew the original you; possibly you will catch some reflection of that long-vanished self in her eyes. If she’s sly, she exploits this. The possibility renders her current, slightly critical gaze (How have you changed?) bearable. This novel takes that kind of inquisitive, proprietary, utterly natural interest and turns it inside out.

The novel begins, and therefore ends, with backyard fireworks on the Fourth of July in Five Oaks, Mich. Metaphorically, at least, fireworks represent brilliant sudden illumination of a kind that “First Light” otherwise avoids; the Fourth symbolizes jubilant release, while the novel has taken for its quiet subject the insistent claims of siblinghood as they exist between a brilliant astrophysicist, Dorsey Welch--”a beauty without innocence . . . because of her eyes. Insomniac attentiveness has darkened them”--and her older brother Hugh, a Buick salesman whose existence has been altogether more modest and fettered. One of his chief fetters has always been to his sister: “My father once told me to watch after her, to take care of her. So I tried. . . . It’s not like being married. It’s this other kind of love. There’s no name for it. Sometimes I think I’ve spent my life watching her, watching over her.”

The novel’s first paragraph consists of a qualification, by Dorsey, of something Hugh has just said. He’s referred to fireworks as explosives, and she fastidiously corrects him: They are toys. Explosions, the author seems to imply, though they occur elsewhere, have seldom rocked these lives; and when, in the backyard, a toy airport constructed by Dorsey’s husband, Simon, is blown apart, Simon’s irreverence infuriates Hugh, who has already, within 20 pages, settled into the novel’s emblem of soberly vigilant love.

“ ‘A revolution is a revolution!’ Simon shouts, lighting the cherry bomb. . . . Hugh raises his head, closes his eyes as the bomb explodes, tearing apart both the car and the Athens airport next to it, and blowing out several cardboard people inside the airport onto the grass. Hugh looks up and sees a flock of sparrows. He wants to see them form into a pattern, an arrow or a letter, but flocks of sparrows never make formations, he reminds himself; they go where they want to.”

Hugh’s recognition--”flocks of sparrows never make formations”--underscores Baxter’s respect for the vagueness and untidiness of perception, and its refusal to fall neatly into compact fictional shapes. This respect, one imagines, underlies this novel’s unorthodox narrative form. The novel harbors a number of striking images that comment on its own backward-seeking structure, as in a scene of Hugh, Dorsey, and their parents viewing an eclipse through photographic negatives. Hugh sees “. . . all of the separate images piled on top of one another, one large distinct collection of the past, and, in the middle of it, the darkening sun, now shaped like a burning three-quarter moon, shining through them.”

This novel’s time, in not going forward, seems curiously deep and still. My impression that this was a really long novel persisted, though, in fact, it’s 286 pages--solid but not massive. The characters’ childhoods are treated with great psychological naturalism: What is enigmatic in those scenes is what would be enigmatic in a child’s eyes, and the reader, who has by then been drawn in very close to Hugh and Dorsey, is confronted with a world mysterious, duplicitous, baffling. Baxter is the author of two previous books, both collections of stories, “Harmony of the World” and “Through the Safety Net.” A story in “Through the Safety Net,” “Talk Show,” is written in simple, short paragraphs, each isolated from the rest of the story by white space, representing the choppy but intense segments of a young boy’s attention and dealing with his fascinated incomprehension of his grandmother’s slow death. It is compelling in the way that the childhood chapters of “First Light” are compelling: The measure of darkness is taken by clear but uncomprehending eyes.

Despite its affection for discontinuity, this novel’s loose ends have a way of sliding together into memorably firm knots. Hugh’s wife and daughters remain shadowy, and Hugh and Dorsey’s parents never quite emerge as individuals, so there is a slightness where the reader could have wished for greater depth, in figures so close to Hugh. In the ending, however, a single gesture is packed with meaning distilled from the previous 285 pages; a crucial instant, frozen in time, concludes this intricately reflective, simply beautiful book.