A memoir by President Donald Trump’s niece due to be published next week describes the president as a pathological narcissist who cheated on the SAT college entrance examination and has embraced “cheating as a way of life” ever since, according to US media outlets who received advance copies of the book.

The book by Mary Trump, the daughter of President Trump’s elder brother, describes how decades of dysfunction and relations with an abusive father moulded the man who would become president into a reckless leader who, according to publisher Simon & Schuster, “now threatens the world’s health, economic security and social fabric”.

The Trump family has been embroiled in a legal battle to halt publication of the book – titled “Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man” – but an appellate court in the state of New York cleared it for release over the objections of Robert Trump, the president’s brother.

Robert Trump has said the book would violate a confidentiality agreement tied to the estate of his father Fred Trump Sr, who died in 1999. Mary Trump is Fred Trump’s granddaughter.

Citing “extraordinary interest” in the book, Simon & Schuster announced on Monday that the book would be released ahead of schedule, becoming available in bookstores on July 14 instead of July 28. Even before its release, the book is on Amazon’s list of top 10 best-sellers.

The New York Times newspaper, which received an advance copy, reported on Tuesday that the book claims Trump paid someone to take the SAT college entrance exam for him when he was in high school in the New York City borough of Queens. His high score on the test helped him secure a spot at the prestigious Wharton School of Business at the University of Pennsylvania.

The book also quotes President Trump’s sister, Maryanne Trump Barry, a retired federal appeals court judge, as having reservations about his fitness for office, referring to him as a “clown”. The sister also expressed amazement at support for him among evangelical Christians in the United States.

“The only time Donald went to church was when the cameras were there,” Mary Trump quotes her aunt as saying during the 2016 campaign. “It’s mind boggling. But that’s all about his base. He has no principles. None!”

White House Press Secretary Kayleigh McEnany on Tuesday called the memoir a “book of falsehoods”.

“It’s ridiculous, absurd allegations that have absolute no bearing in truth. Have yet to see the book, but it is a book of falsehoods,” she told reporters outside the White House.

Mary Trump, a clinical psychologist, claims the president meets all the criteria for being a narcissist, but writes that even that diagnosis does not capture the full array of his pathologies.

“The fact is,” she writes, “Donald’s pathologies are so complex and his behaviors so often inexplicable that coming up with an accurate and comprehensive diagnosis would require a full battery of psychological and neurophysical tests that he’ll never sit for.”