The Rock Behind Rockefeller : ABBY ALDRICH ROCKEFELLER, <i> By Bernice Kert (Random House: $30; 537 pp.)</i>

Bernice Kert demonstrated in “The Hemingway Women” an eye for offbeat biography. This time she brings us Abby Aldrich Rockefeller, the patrician rebel who breathed humanity into the somber Rockfeller clan, advanced the causes of working women and minorities, and by founding New York’s Museum of Modern Art left a lasting imprint on the American cultural scene.

Kert’s penetrating close-up captures not only a remarkable personality but the suffocating nuances of post-Victorian matrimony; women readers in particular will relish Abby’s refusal to be pigeonholed. If Kert occasionally sacrifices pace to detail, a rich collection of personal letters--assiduously researched, adroitly dovetailed--soon puts her narrative back on track.

The daughter of a prosperous U.S. Senator, Abby Aldrich in 1901 married into a family whose assets made hers look like petty cash. Her husband, John D. Rockefeller, Jr., was the only son of the legendary oil baron, raised in isolation, robotically programmed to continue his father’s global benefactions. A billionaire before he was forty, John staggered under the double burden of riches that all but defied expenditure, and a stern Baptist religiosity that occupied his conscience every waking moment and through many a sleepless night. His personal needs neglected, he was a timid, insecure suitor who predictably became a remote father and an exacting, controlling husband.

But Abby saw--and loved--the tormented boy beneath; gave him the physical fulfillment and intellectual companionship he desperately craved; and somehow satisfied her own yearnings for expression while walking the delicate tightrope of their relationship.

Abby Aldrich Rockeller’s immersion in modern art began on a European gallery tour with her discovery of the German Expressionists. Captivated by the intensley personal credo of such pioneers as Kathe Koilwitz and Lyonel Feininger, she plunged into serious collecting. Few modernists of talent, at home or on the Continent, escaped her eager scrutiny, although at first only a relative handful, thanks to John’s watchful eye, wound up in her collection: John Sloan, Winslow Homer, Arthur B. Davies, and in minor manifestations Picasso and Matisse.

But the pleasures of fine art, Abby felt, should not be reserved for the wealthy. In the spring of 1929 she rounded up a coterie of like-minded women, persuaded a personable Ivy League gallant to front for them in a male-dominated art milieu, and set about the arduous task of raising money for a modern art milieu, and set about the arduous task of raising money for a modern art museum. Step by step, with Abby directing from the sidelines, the operation inched forward, from a temporary base on West 57th St. to an opening in November with a loan exhibit from France.

Meanwhile Abby hobnobbed happily with William Zorach and the sculptor’s radical wife Marguarite, entertained Matisse at dinner, and engaged the fiery Diego Rivera to create a huge mural for Rockefeller Center--all under the increasingly critical gaze of her husband. John had no patience with modern art; Jackson Pollock’s tortuous drippings offended his sense of no-nonsense discipline. To spare John irritation, Abby tucked away her prints and canvases in a lofty ghetto of their nine-story town house.

Yet even this opposition she overcame; John gradually increased her art allowance. Her final victory in their tug of war over the MoMA came when he not only helped to pay off its sizable mortgage but turned over a prime piece of midtown real estate to accommodate its outdoor sculpture garden.

Kert makes a somewhat sluggish start, reeling off ancestral names and dates with a kind of clinical detachment. She hits her stride as the newlyweds come into focus: John, if not cast in the ruthlessly acquisitive mold of his father, nonetheless the personification of cautious capitalism; Abby instinctively open-minded, a champion of the underprivileged, drawn to art and artists. Where John was shy, reclusive, harnessed to his Bible classes and his inheritance, Abby was avid for life, trained in the social graces as her father’s frequent hostess in Washington, receptive to people of every economic class and ethnic origin. How to pursue her passion for women’s rights while wrapped in the purse strings of a pathetically neurasthenic billionaire?

Her one weapon, astutely but never callously wielded, was her husband’s near-absolute emotional dependence. For all of his domestic tyranny, John was a fragile creature who shrank from contact with his five sons and was hopelessly at odds with his one daughter. Jealous of their children and his wife’s other interests, he plotted constantly to keep her at his side.

A large woman of stately bearing, Abby was hardly the dainty sprite intimated in John’s courtship letters. What he apparently found irresistible was her tireless joie de vivre. yet even with her he could not always surmount his ingrained formality. Although the two lived under the same roof and shared what Kert assures us was an active bed, when John raised his wife’s living allowance he did so in a terse businesslike memorandum.

Kert has had the good judgment to explore the family correspondence and--and the good fortune to strike gold there. Abby’s letters convey the woman: To her three eldest sons in 1923: “One of the greatest causes of evil in the world is race hated or race prejudice...The two peoples...who suffer the most in this country are the Jews and the Negroes.”

To Charles Eliot, former president of Harvard, in 1925 affirming her membership on the Board of the Rockfeller-backed International House in Manhattan: “I am deeply interested in the problem of making better international relations. If the future leaders of the world are brought together while they are still young, (perhaps) there will be an understanding that will make world peace.”

Abby spearheaded the construction of the low-cost for-women-only Dodge Hotel in the capital, and established a community house for immigrant employees near the Jersey Standard plant in Bayonne. And she opened her purse regularly to settlement houses, the United Jewish Appeal and Margaret Sanger’s American Birth Control League.

Kert compounds a dearth of vivid physical description, cluttering her landscape with lifeless detail (“on July 10 (Abby and John) checked into the Hotel Crillon in Paris for eight days”) that could have been winnowed out by a more vigilant editor.

But Abby Aldrich Rockefeller emerges intact. “Had she been a man,” according to the architect Philip Johnson, “she could have commanded an army;” under other circumstances, her powers of persuasion might have landed her in the Senate. Yet her greatest strength was her character. If Abby had been married to one of the Rockfeller oil field hands instead of to the kingpin himself, one feels she would have held to the same principles and promoted the same values. That she happened to run the household and influence the decisions of the wealthiest man in America was a windfall for the rest of us.