In Novel by Mayor’s Daughter, Hints of Family Life

But perhaps no single sphere of Mr. Bloomberg’s life has been as thoroughly walled off as his family. They occupy a realm that seems, even after the mayor’s 10 years in public life, unknowable.

Now, however, a longtime member of the mayor’s inner circle is breaking ranks, in a fashion, and offering a portrait of a Bloomberg-like household that is intimate and unvarnished, riven by sibling rivalry, resentments over money and a Type A father bent on conventional success for his daughters.

The author of this insider’s account: Mr. Bloomberg’s younger daughter, Georgina.

And she has cracked open the door in perhaps the only way a Bloomberg ever could. Her book is, strictly speaking, a work of fiction. There is little doubt, however, about who has inspired the main characters.

In “The A Circuit,” a young-adult novel by Ms. Bloomberg that just arrived in bookstores, the father figure, Rick Aaronson, is a blunt-talking Wall Street billionaire who lives in a Manhattan town house and “owns half of New York.” His older daughter, Callie, is an Ivy League graduate with a passion for politics. And his younger daughter, Thomasina, or Tommi, is an award-winning equestrian who chafes at her father’s expectations of a traditional career.

Mr. Bloomberg, of course, earned his fortune on Wall Street, lives in a Manhattan town house and is notorious for his candor. His older daughter, Emma, graduated from Princeton and went on to work for him at City Hall. And his younger daughter, Georgina, or George, is a professional horse jumper who has spoken openly of struggling to prove to her father that riding was a serious profession.

“She wasn’t afraid to say no to her father,” Tommi explains in the book, “even if half of Wall Street was.”

When she announces she will pursue a career in riding, rather than something practical, like the law, her father curtly tells her to “grow up.” He scoffs, “Nobody does that.”

As it happens, the real Mr. Bloomberg has occasionally grumbled about his daughter’s unorthodox profession, which, unlike his, tends to burn through as much money as it generates. Show horses can cost hundreds of thousands of dollars, and Ms. Bloomberg owns at least six, which she keeps on estates in North Salem, N.Y., and Wellington, Fla.

“How you make a living doing that, I don’t know,” the mayor complained last year when asked about his daughter’s riding life. “But we’ll leave it up to George.”

If anyone would break the unwritten rules of Bloombergland, it would be Georgina, who has never fit the mold of the dutiful daughter. She was never a big presence in her father’s campaigns, told a documentarian that “having the last name Bloomberg sucks” and startled her father’s aides when she spoke publicly last year about being dumped by her boyfriend.

The idea for the book came from an agent, said a spokeswoman for the publisher, Bloomsbury, which offered Ms. Bloomberg a two-book contract and put her with a co-writer, Catherine Hapka.

Unlike most authors, who clamor for news media attention, Ms. Bloomberg did almost no promotion and, after a single sit-down interview the day her book was published, hopped on a plane to Bermuda for a vacation with friends. (Her waterfront house is a few hundred feet from her father’s.)

“My publicist told me I’m the only author who leaves town the day her book comes out,” she said during the interview, in a Manhattan restaurant.

Ms. Bloomberg said she saw the book as a glimpse into her life in the horse-show world, not necessarily a faithful portrayal of her family.

But in between long pauses and knowing giggles, Ms. Bloomberg, 28, said that she drew heavily from her own experiences to write the book, and that she strongly related to the novel’s main character, the young heiress Tommi.