Donna Tartt’s The Secret History was the rare first novel that is both a critical and a commercial success. This bestseller (it’s been translated into 24 languages) was a brilliant psychological study welded onto the solid plot of a thriller. Set at a thinly disguised Bennington College, The Secret History chronicles the fatal attraction of Richard Papen, a provincial Californian, to a wealthy, intellectual coterie of classics students who are led to murder by their study of ancient Greek philosophy. It’s a deftly plotted murder mystery with an almost Fitzgeraldian portrait of how the rich and talented seduce and destroy the lesser mortals in their orbit.

In her second novel, The Little Friend, Tartt has once again written a kind of murder mystery, grafting her psychological explorations onto a swift, cinematic story. And once again she shows an uncanny understanding of guilt, how it grows from the moment of the crime to choke the rest of the criminal’s days and nights. But in setting and character, The Little Friend is diametrically opposed to its predecessor. We’re in Tartt’s native Mississippi, where the naive collegian has been replaced by a disturbingly cunning child. And the presiding spirit is Faulkner, along with his gothic, class-and-guilt-ridden South.

In 12-year-old Harriet Cleve Dufresnes, Tartt creates an unsentimental child heroine who combines an adult’s enterprise and freedom with a child’s intolerance of ambiguity and compromise. Tartt remembers that childhood is a time not of innocence but of vicious moralizing:

She did not care for children’s books in which the children grew up, as what " growing up " entailed (in life as in books) was a swift and inexplicable dwindling of character; out of a clear blue sky the heroes and heroines abandoned their adventures for some dull sweetheart, got married and had families, and generally started acting like a bunch of cows.

An undwindled ferocity fuels Harriet’s quest to track down and punish the murderer of her brother Robin, who was killed in his back yard in broad daylight while she was an infant. She has no memory of her brother, but the aftershocks of his brutal murder — he was hanged in a tree with an electrical cord, as his family gathered for Mother’s Day — still rattle her life. Her father has moved out to live with his mistress in Nashville; her mother is a tranquillized specter, neurotically hoarding old newspapers; her older sister Allison, who witnessed the killing, has retreated into moody vagueness. The dominating presences in Harriet’s life are her grandmother Edie — from whom she inherits her powerful will — and her black housekeeper, Ida. It’s this house of shadows, with its lingering boredom and exhaustion and fear, from which Harriet yearns to escape.

She decides that the only way to win her freedom is to solve Robin’s murder, a case the police have long since abandoned. With troubling alacrity, she turns a casual remark of Ida’s into the absolute conviction that the culprit is Danny Ratliff, one of a family of violent and deranged hillbillies. " The only thing that gave Harriet a sense of purpose was the idea of Danny Ratliff. . . . The despair of her house was the work of his hand. He deserved to die. " The all-male Ratliffs — along with Danny, there’s the speed-freak Farish, the Jesus-freak Eugene, and the gently retarded, Benjy Compson–like Curtis — are the mirror image of the all-female Cleves with their fading plantation gentility.

In pursuit of Danny, Harriet and her best friend, the sunny and shallow Hely Hull, embark on a series of terrifying adventures. As with The Secret History, many readers will enjoy The Little Friend less for its psychology than for its action set pieces, which are as effective as anything in Stephen King: Hely trapped in the Ratliffs’ lair with boxes of poisonous snakes, Harriet struggling to escape drowning in an abandoned water tower. But unlike a conventional thriller, The Little Friend denies you the satisfaction of moral clarity and full solution. In other hands, Harriet would be a cute, precocious child detective who saves the day. For Tartt, she is a way of exploring the thin boundary between heroism and cruelty, courage and delusion, purity and vengefulness. And Harriet’s victory, her passage to adulthood, is only the dreadful recognition of this ambiguity: " She had almost been a hero. But now, she feared, she wasn’t a hero at all, but something else entirely. "

Donna Tartt reads from The Little Friend on Thursday November 7 at 6 p.m. at the Harvard Book Store, 1256 Massachusetts Avenue in Harvard Square. This event is free; call (617) 661-1515.