Trump mocked his father as he started succumbing to Alzheimer's, according to his niece's upcoming tell-all

• President Donald Trump's niece, Mary L. Trump, is due to release a tell-all book about her uncle on July 28 through Simon & Schuster.
• Among the details in the book is how Donald Trump "dismissed and derided" his own father when he started succumbing to Alzheimer's in the 1990s, according to a newly released description of the book.
• Mary Trump is the daughter of the president's older brother Fred Trump Jr., who died, in 1981, from alcoholism.
• When Fred Trump Sr. died, in 1999, Mary Trump and her brother got into a testy legal battle with Donald Trump and two of his siblings over their inheritance.

Donald Trump mocked his father, Fred Trump Sr., as he started succumbing to Alzheimer's disease in the last six years of life, according to new details from a forthcoming tell-all from the president's niece.

Mary L. Trump, the 55-year-old daughter of the president's late older brother, Fred Trump Jr., is due to publish "Too Much and Never Enough: How My Family Created the World's Most Dangerous Man" on July 28.

The book is a "revelatory, authoritative portrait of Donald J. Trump and the toxic family that made him," according to a description of the book posted online Monday by publisher Simon & Schuster.

As a clinical psychologist, Mary Trump "has the education, insight and intimate familiarity needed to reveal what makes Donald, and the rest of her clan, tick," the description added.

Among the family drama that Trump is set to spill in the book is how the current US president "dismissed and derided" his father "when he began to succumb to Alzheimer's," which he was diagnosed with in 1993, according to Simon & Schuster.

Mary Trump, who now works as a life coach, has largely stayed out of the limelight.

But she did get into a protracted legal battle with her uncle after her grandfather died, in 1999, leaving her and her brother a substantially smaller inheritance than his other grandchildren.

Mary and her brother, Fred Trump III, contested the will, causing Donald to cut them off the family's medical-insurance plan, which their grandfather had provided free of charge while he was alive.

This was especially difficult for Fred III, who had a son with cerebral palsy who required extensive care.

Mary Trump bashed her uncle and two of his siblings in a 2000 interview with the New York Daily News about the squabble, saying her "aunt and uncles should be ashamed of themselves," referring to Donald, Robert, and Maryanne Trump.

The issue over the inheritance was later settled and details of the deal were not released.

Sources previously told The Daily Beast that Mary Trump would also identify herself in the book as the primary source of The New York Times' 2018 exposé on the president's finances, and say that she provided confidential tax documents to help the newspaper's investigation. The Times declined to comment on her claim.

The story showed how Donald Trump used shady tax schemes to transfer more than $400 million of his father's money to keep his businesses afloat, countering his claim that he was a self-made man.

News of Mary Trump's book comes as former national-security adviser John Bolton is also due to publish a book next week. The White House had previously delayed the publication of the book, and Bolton said the president was "determined to prevent publication" of it.

• Read more:
• Trump's niece is publishing a tell-all book that says she leaked tax documents to help The New York Times investigate the president's finances
• Trump in new interview admits 'regret' over how he treated his late brother Fred Trump Jr., who struggled with alcoholism
• The life of Donald Trump Jr., who once lived out of a truck, didn't speak to his father for a year, and made waves on a book tour with his girlfriend
• Trump's 81-year-old sister is a federal judge who Bill Clinton nominated to the US Court of Appeals