On July 26, 2006, New Yorkers awoke to headlines that Brooke Astor’s grandson had asked the courts to block his own father, Anthony Marshall, from handling her estate.

Mrs. Astor, by then a fragile 104, was sleeping on a urine-stained couch, the grandson charged. She was wearing a tattered nightgown and eating pureed peas and oatmeal.

Mr. Marshall was later cleared of elder abuse. Yet “while this news might elicit both outrage and sympathy,” Frances Kiernan writes in her biography “The Last Mrs. Astor,” “they had the cumulative effect of casting a shadow over decades of important contributions, threatening to permanently alter the way New Yorkers looked at the lady who had once reigned as de facto queen.”

Beginning in the 1960s, Nelson Aldrich observed last December in the New York philanthropy magazine Contribute, Brooke Astor “set a standard for would-be philanthropists that is now emulated, though seldom with due credit, by ‘nouveaux riches’ benefactors all over the country.” Mrs. Astor, he wrote, gave money locally, not nationally; personally, not anonymously; practically, not grandly; and in a targeted, not scattershot, manner.

Would that Frances Kiernan — a former fiction editor at the New Yorker and the author of a biography of Mary McCarthy — had so clearly outlined the significance of this extraordinary woman. Instead, readers have to weed through a mass of detail to figure out why Mrs. Astor was more than the society-page footnote some mistake her for.

Born in 1902 to a comfortable but hardly rich Marine Corps captain and his wife, Brooke Russell grew up in Washington, Panama, and Peking.

Married at 17 — disastrously — she divorced and, before it was socially acceptable, lived independently in New York. Husband no. 2 was her great love, the stockbroker Charles “Buddie” Marshall.

When he died, in 1952, she infamously waited just 11 months before marrying no. 3, the multimillionaire Vincent Astor, whose taciturn, suspicious personality stacked up poorly against the charms of the late, great Buddie.

Did she marry for money? Frances Kiernan goes easy on her subject throughout the book. She quotes the patrician novelist Louis Auchincloss’s line: “Of course she married Vincent for the money. Only a twisted person would have married him for love.”

At his death, in 1959, Vincent Astor left an estate estimated at $134 million — $900 million in today’s terms.

The money allowed his widow, who was already an accomplished writer (eventually of five well-reviewed books), to answer her true calling as a philanthropist.

Over time, she applied the Vincent Astor Foundation’s $67 million endowment to the improvement, and sometimes the salvation, of such New York institutions as the Bronx Zoo, where she underwrote two natural habitats; the Metropolitan Museum of Art; the New York Public Library, and the impoverished but architecturally rich Brooklyn neighborhood of Bedford-Stuyvesant, whose restoration became a prototype for community-based initiatives.

She was a model of the hands-on philanthropist, known for her on-site visits (always in hat and gloves), her pointed remarks, her facility for dominating the cigar-smoking, white-male charity boards that competed for her membership.

As late as her 90s, Mrs. Astor moved stylishly through many worlds, hosting balls that lasted deep into the night. “Darlings, it doesn’t matter,” she shrugged at one after dropping an impossibly valuable emerald brooch on the floor. “We’ll get it later — in the morning.”

“The Last Mrs. Astor” (the early-1900s socialite Caroline Schermerhorn Astor was the first) is best when Ms. Kiernan is doling out gems gleaned from debriefing Astor’s upper-crust pals: how Brooke married Vincent only after the millionaire’s soon-to-be ex-wife vetted her.

How she extracted a promise of faithfulness from Buddie by pitching her Cartier wedding ring out the limo window after he’d flirted. How she advised a young bride that the secret to a happy marriage was a little fling on the side.

In 1966, New York’s Landmarks Conservancy designated her one of the city’s “living monuments.” She hated the term. She would have preferred to continue swimming her daily laps, lunching at La Grenouille — and signing checks. But age caught up.

A pathetic old woman in a soiled nightgown? As this biography makes clear, in her final act deserves so much more.

“The Last Mrs. Astor: A New York Story” is published by Norton (307 pages, $24.95).