Only three months into Justice Ketanji Brown Jackson’s first Supreme Court term, she announced a book deal negotiated by the same powerhouse lawyer who represented the Obamas and James Patterson.

The deal was worth about $3 million, according to people familiar with the agreement, and made Justice Jackson the latest Supreme Court justice to parlay her fame into a big book contract.

Justice Neil M. Gorsuch had made $650,000 for a book of essays and personal reflections on the role of judges, while Justice Amy Coney Barrett received a $2 million advance for her forthcoming book about keeping personal feelings out of judicial rulings. Those newer justices joined two of their more senior colleagues, Justices Clarence Thomas and Sonia Sotomayor, in securing payments that eclipse their government salaries.

In recent months reports by ProPublica, The New York Times and others have highlighted a lack of transparency at the Supreme Court, as well as the absence of a binding ethics code for the justices. The reports have centered on Justice Thomas’s travels and relationships with wealthy benefactors, in addition to a luxury fishing trip by Justice Samuel A. Alito Jr. with a Republican megadonor and the lucrative legal recruiting work of the wife of Chief Justice John G. Roberts Jr.

The book deals are not prohibited under the law, and income from the advances and royalties are reported on the justices’ annual financial disclosure forms. But the deals have become highly lucrative for the justices, including for those who have used court staff members to help research and promote their books.

Earlier this year, Justice Jackson confirmed her publishing agreement with an imprint of Penguin Random House for her forthcoming memoir, “Lovely One.” But like her colleagues, her first public acknowledgment of the financial arrangement behind the deal is likely to be in her future annual financial disclosures. The New York Times learned the rough dollar amount of her advance, a figure that had not previously been disclosed, from people familiar with the deal.

Justice Jackson did not respond to questions about the deal sent to her through a court spokeswoman.

Justice Sotomayor has received about $3.7 million total for a memoir documenting her path from a Bronx housing project to the federal bench and her children’s books. The justice’s administrative court staff urged organizers of events where her books were sold to buy more copies, according to a recent report in The Associated Press, which cited public records.

A New York Times review of acknowledgments in other books showed that some justices thanked staff for its work compiling the books. Justice Gorsuch, for example, thanked three staff members for assisting him on his 2019 book, singling out one for her “amazing editorial help” and praising two others for their “eagle eyes.”

Justice Thomas, in his memoir, thanked a staffer in the Supreme Court Library who “worked tirelessly to track down even the most obscure facts and documents based on my faintest recollections.”

The justices are required to abide by federal regulations setting limits on outside income, similar to members of Congress and high-level officials in the executive, legislative and judicial branches. There is a cap of roughly $30,000 on outside pay for the justices, which mainly applies to teaching and other honorariums.

But books are not included in the caps, making them enticing as supplements to the justices’ federal annual salaries, now $285,400 — an amount that far outpaces the pay of most Americans, but is much lower than the justices are likely to make in private law practice.

The book deals of current justices are not unique. Scotusblog, a website devoted to covering the Supreme Court, in 2012 posted a list of 353 books written or edited by justices since 1776, from autobiographies and poetry to legal theory and ruminations on war.

To some judicial ethics watchdogs, the books are low on the list of ethics concerns.

“Those are really interesting and uplifting American stories,” said Gabe Roth, the executive director of Fix the Court, an organization that has been critical of the court’s transparency. “They absolutely should be out there, in the voice of the ones who lived them.” He added, “If you can make a little money off that, I don’t begrudge them.”

But some experts said the book deals — and the lack of transparency and clear guidelines around them — are another sign that the nation’s highest court is out of step with other federal judges and government officials.

“The Supreme Court is in the Stone Age of ethics,” said Kedric Payne, the vice president of the Campaign Legal Center and a former deputy chief counsel for the Office of Congressional Ethics.

Mr. Payne said that other public officials routinely deal with how to handle ethics around books. Congress has specific rules around book deals. The executive branch also has ethical oversight. The court could make a similar choice, he said, but has chosen not to.

“These are not novel issues,” Mr. Payne said. “The Supreme Court has decided not to be clear in how they handle ethics.”

One area of particular concern, experts said, is how justices have used court resources to bolster their book ventures, which is paid work that falls outside the scope of their court work.

“I think that’s troubling because staff will have great difficulty saying no,” said Amanda Frost, a law professor at the University of Virginia who specializes in legal ethics. “It is nonjudicial activity, involving money coming to the justice.”

Other federal judges are bound by the Code of Conduct for United States Judges, which lays out guidelines and rules for judicial conduct. One of the tenets focuses on a judge’s use of court resources for outside activities. The rule says that “a judge should not to any substantial degree use judicial chambers, resources or staff to engage in extrajudicial activities.”

A spokeswoman for the court, Patricia McCabe, declined to say whether the staffers were paid for the work on the books. The court did not respond to questions about whether government employees thanked by the justices in acknowledgments had worked on their books during official time or if they had some other arrangement.

The Times reported earlier this year that justices have heavily relied on their staff to support other paid outside work, including teaching, despite a judicial advisory opinion — which the justices say they voluntarily follow — that staff members should not help “in performing activities for which extra compensation is to be received.” Records reviewed by The Times showed how staff for Justices Thomas, Gorsuch and Brett M. Kavanaugh took on significant labor for their classes at George Mason’s Antonin Scalia Law School, including organizing class materials and student papers, managing student visits and coordinating guest lectures.

The court issued a recent statement in response to The Associated Press about books deals, writing that the judicial code of ethics “encourages judges to stay connected to community activities and to engage with the public, including by writing on both legal and nonlegal subjects.” The justices routinely travel and speak to various audiences, and staff members “play an important role in assisting on issues of ethics, travel and security.”

The records obtained by The Associated Press show the extent to which Justice Sotomayor’s court staff assisted in arranging her appearances and weighing in on book sales, pushing organizers of events at host colleges and libraries to buy more copies.

“For an event with 1,000 people and they have to have a copy of ‘Just Ask’ to get into the line, 250 books is definitely not enough,” one aide to Justice Sotomayor wrote to staff of the Multnomah County Library in Oregon before a visit in 2019. “Families purchase multiples, and people will be upset if they are unable to get in line because the book required is sold out.”

The same aide stayed on top of book sales and autograph lines at an event at Portland Community College, the records obtained by The Associated Press showed. “Is there a reminder going out that people need to purchase a book at the event or bring a book to get into the signing line?” the aide wrote. “Most of the registrants did not purchase books.”

The court, in its statement, said chambers staff makes recommendations on book purchases before an event “based on the size of the audience so as not to disappoint attendees who may anticipate books being available at an event.” Justice Sotomayor’s staff also followed guidance, the court added, that “there should be no requirement or suggestion that attendees are required to purchase books in order to attend.”