Donald Trump's a phony, a liar and above all else -- a cruel and dangerous egomaniac now in charge of America's well-being ... according to Mary L. Trump, the President's niece.

Excerpts from Mary's new book, "Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man" have been released, and in it, she alleges her uncle was the original Felicity Huffman ... and not for acting.

Mary claims Trump cheated on a college entrance test -- the SAT -- by paying someone else to take it for him in Queens, NY ... and that high score helped him get into Wharton Business School at Penn.

Trump's niece, who is a clinical psychologist, also says her uncle developed "twisted behaviors" from an upbringing in the rich and notorious real estate family led by his father, Fred Trump Sr. ... which led to him implementing "cheating as a way of life."

She says his own sister -- Maryanne Trump Barry -- referred to him as a "clown" when he began running for prez, and was baffled by Christians supporting him. According to the book, Maryanne said of her brother ... "It’s mind boggling. But that’s all about his base. He has no principles. None!"

As for 45's cruelty and narcissism ... Mary alleges, before her father -- Donald's older brother -- Freddy, died from an alcohol-induced heart attack in 1981, the family sent him to the hospital alone. Donald, she claims, went to the movies instead.

The tell-all book's publication is being challenged by the Trump family -- who cite a confidentiality agreement from nearly 20 years ago -- but so far ... a judge is NOT blocking its planned release next week. Advance copies have already gone out to several media outlets.

Mary has been estranged from the family for years and is the first Trump member to break ranks and divulge alleged family secrets.

The judge is expected to rule soon on whether or not she violated her confidentiality agreement.