During the court contest last year about the guardianship of Brooke Astor, an unsettling, though charitable, thought loomed over the proceedings: In her diminished mental and physical state, with friends removing her from the care of her son, Mrs. Astor, 104 at the time, would be better off dead.

Biographers can’t control such things, but Mrs. Astor, now 105, is most certainly better off for the biography that Frances Kiernan has written about her, “The Last Mrs. Astor” (Norton, 256 pages, $24.95).

The book is an admiring account of Mrs. Astor’s life, showing how her upbringing, three marriages, leisure pursuits, and work as a writer and editor prepared her for her greatest act: 30 years of distributing her late husband’s wealth to hundreds of New York nonprofit organizations.

The book is not a hastily assembled response to events of the past year. Ms. Kiernan, a former editor at the New Yorker and the author of a biography of Mary McCarthy, first met Mrs. Astor in 1999, with the idea of helping her write a third installment of her memoirs. That book didn’t come to pass, but two years later, Ms. Kiernan decided she would take on Mrs. Astor as a subject of a biography, prompted by the terrorist attacks of September 11, 2001. “I began to think how much in need the city was of someone like Brooke Astor,” Ms. Kiernan writes

Ms. Kiernan has focused on Mrs. Astor’s generosity to the city during the fiscal crisis of the 1970s. It was during this period that she left her mark on New York, and those gifts make her most deserving of a full-length biography — even though the amounts she distributed were small compared to those of other donors in the old money pantheon, and even though today, her style of philanthropy might be considered quaint and undisciplined.

Ms. Kiernan sees Mrs. Astor as a “hands-on philanthropist,” a role she embraced and for which her early life prepared her. Before she began spending time with the poor people who benefited from her gifts, she had during World War II cared for soldiers at Halloran Hospital on Staten Island, to which she traveled by bus, subway, and ferry. Managing the egos of socialites and celebrities she covered as an editor of House & Garden equipped her to deal with the egos on the boards of the Metropolitan Museum of Art and her own foundation.

She became famous for her good works because of her last name, the force of her personality, and the amenability of the press. She did not always have it easy; there were often financial strains in her first two marriages, and when she became a widow in her 50s her circumstances were worse than they’d ever been, a factor that helped lead her to a marriage in less than a year to the well-off, though unattractive and not-much-liked Vincent Astor.

Ms. Kiernan’s impressions of Mrs. Astor draw on interviews with dozens of people who knew her in her more active years, research at the New York Public Library, press clips, an unpublished interview conducted in 1997, and Mrs. Astor’s own writings, including two memoirs, “Patchwork Child,” published in 1962, and “Footprints,” published in 1980.

Ms. Kiernan corrects some inaccuracies presented by Mrs. Astor in her own tellings, but she is not out to uncover unknown truths. There are, thankfully, a few provocations.

At one point Kiernan suggests that Mrs. Astor’s sense of style was more owing to the good judgment of her French maid, than to her own, and that the maid would routinely return clothing Mrs. Astor had purchased.

Ms. Kiernan herself interviewed Mrs. Astor, around the time of her 100th birthday, but didn’t get too much from her. A visit in 2005 seems to have assured the author that the proper conclusion of this biography would be detailed accounts of the celebrations of her 100 th birthday, which had taken place in 2002.

“From what I could see that afternoon, Mrs. Astor’s story was over, awaiting only her death to provide a quiet coda,” the author writes.

It’s a terrible disappointment then that Ms. Kiernan effectively kills off her subject at the age of 100. She does tell about Mrs. Astor’s decline in old age: her inability to hear or comprehend events, her bowing out early from a celebration at the New York Public Library when she was made a Literary Lion, and how she lives her days at Holly Hill in Briarcliff Manor without the walks and friends she relied on through most of her life.

But there are details about her care — namely, what her grandson, Philip Marshall, and friend Annette de la Renta observed that made them take legal action, and most importantly, how the world responded to the news — that the fewer than 10 pages on the guardianship fight do not recount.

Ms. Kiernan points out how Mrs. Astor would often talk about how she grew up in a gentler time. It seems that Ms. Kiernan herself wishes she were in that time, instead of the present, the one in which the most famous socialite of the day is Paris Hilton, who acts in reality television shows and serves jail time, and elderly women can lose all the pleasures of their life yet continue to live.