Mockingbird South of the Border : FIRST CONFESSION, <i> By Montserrat Fontes (W. W. Norton: $18.95; 320 pp.)</i>

“First Confession,” Montserrat Fontes’ elegiac and powerful first novel, is a coming-of-age story told in the voice of a 9-year-old girl named Andrea, the willful and indulged child of a wealthy Mexican father and a willowy, blond American mother. Both in tone and content, it brings to mind Harper Lee’s “To Kill a Mockingbird” and Carson McCullers’ “A Member of the Wedding.”

The year is 1947, the place a small town on the Texas-Mexico border. The events in the story take place over the course of one summer, the summer in which Andrea and her cousin Victor, who has come to live with her family temporarily, are preparing for their First Communion.

If this sounds like a setup for an ordinary, bucolic account of childhood, it’s misleading, for this is a novel with a dark undercurrent of drama, one in which questions of sin and redemption are raised, and issues of class and cultural difference examined. Borders of all kinds are explored--the literal border dividing Mexico and the United States, the border between wealth and poverty, between Indian mysticism and Catholicism, and, perhaps most important, the border between innocence and knowledge, childhood and the adult world, naivete and the awareness of carnal sin.

At the beginning of the novel, the reader is introduced to Andrea’s family. Her grandfather is revered as an important figure for having brought the railroad to town. Although the family lives in considerable luxury on the Mexican side of the border, Andrea’s mother, who has mastered neither the language nor customs of Mexico, prefers shopping on the American side. Andrea’s father, “a beautiful wild man, powerful, lawless,” fits Octavio Paz’s description of the Mexican male who shuts himself away in harsh solitude, and is both “barbed and courteous.” He also drinks heavily, and has a passionate affection for the great bullfighter Manolete.

When cousin Victor arrives for the summer, he brings with him his own personal maid, Alicia, who, because she is light-skinned and from a different region of Mexico, fits uneasily into the household staff, most of whom are from the area near the river. The river, which divides the town and around which the poor are clustered in shanties, functions something like Boo Radley’s house in “To Kill a Mockingbird.” The children are ineluctably drawn to its danger and mystery.

Andrea and Victor are no ordinary children. They are distinguished not only by their privileged lives but also by their penchant for mischief: “Those first days of that summer Victor and I were particularly destructive. Sometimes our goal would be to do fifty bad things before lunch. . . . The club our families belonged to offered the greatest opportunity. Stealing glasses from the bar and breaking them in the parking lot, leaving faucets running, stuffing toilets with wash towels . . . standing by the side of the road yelling obscenities to people who drove by was thrilling. We loved stealing, especially from Popeye’s, a store near their house.”

Popeye’s is owned by Don Pancho, “a dark, thick, smelly man,” and his “fleshy” wife Armida. One day, the children discover stairs that lead to a loft above the store, where a space between the floorboards allows them to spy on the rooms below. There they discover “sin” by witnessing sexual acts. Armida “sells touches” to high school boys who come to the store while her husband takes his daily siesta. “Watching Armida sell touches,” says Andrea, “became our favorite thing to do.”

The children reason that since Armida is a mala mujer --a bad woman--the money she receives from the boys also must be bad. But they will make it good. They decide to steal the money, which is stashed in a coil of rope. They intend to use it to buy presents for the poor children who live down by the river. But the theft of the money instead sets off a tragic chain of events. The children loose upon the world the demons of violence and revenge. A curse, they are convinced, has descended upon their house. The ensuing deaths, beatings, expulsions, suicide and familial ruptures occur against the background of Andrea and Victor’s increasing obsession with what constitutes good and evil. How can they do penance for such damage? What must they confess?

What begins so innocently has horrifying consequences, and Andrea comes to understand there are sins too great for absolution, sins that become “deposited in the vessel we carry inside our souls, the cold lonely vessel that holds pain and secrets that cannot be sharedwith anyone. . . . At an early age I learned what it was to be condemned to silence, to spiritual solitude.” She comes to know the intricate webbing of fear and religion, sin and guilt. Three events occur simultaneously, as if to mark the end of her innocence: Her hair is cut, the beloved Manolete is killed, and she makes her first confession, although she cannot bring herself to tell the priest the truth.

What Fontes does so well is explore the spiritual terrain of childhood, revealing the layered forces that create the mystical, and often confused, notions that children develop around concepts of God and sin. Her writing is free of sentimentality, her narrative voice strong, consistent and clear.

Fontes’ own roots are sunk deep in Mexican history, though she now lives near Los Angeles. She writes about these two cultures--Mexican and American--with a great understanding about what makes us so different.

As a portrait of a town caught on the cusp of these two cultures, and a family foundering on the same rocky terrain, “First Confession” is a most original and darkly powerful work.