She served as the senior adviser to the president and submitted her resignation in January 2018 claiming that she'd never vote for him again. In the book, she paints Mr Trump as scattered, self-absorbed, misogynistic and insecure.
Apparently, she's got some secretly recorded audio of Trump that she's already shared with her close friends and will release in her book, which will be called Unhinged.
Asked about the allegations at a photo-op with the group Bikers for Trump on Saturday in Bedminster, New Jersey, the president said: "Lowlife".
"I didn't want to believe it", she writes.
In excerpts from the book entitled Unhinged, which were first reported by the Guardian, Manigault Newman claimed the president had used the "N-word" repeatedly, and also used racial slurs to refer to George Conway, the half-Filipino husband of the top White House aide Kellyanne Conway.
"It had finally sunk in that the person I'd thought I'd known so well for so long was actually a racist", she wrote, according to The Guardian.
But when pressed about her version in the book, Manigault Newman told to NPR: "I heard the tape". Trump starred on the reality-TV show for years.
Along with her "race war" comments Omarosa said, laughing, that "oh yeah, there's some people gettin' it on in the white house".
She also alleges that Trump has exhibited signs of a "mental decline that could not be denied" and says she even printed a study linking Diet Coke consumption to dementia and strokes and left it on his desk, according to a copy of the book obtained by the Associated Press.
"Instead of telling the truth about all the good President Trump and his administration are doing to make America safe and prosperous, this book is riddled with lies and false accusations", said White House press secretary Sarah Huckabee Sanders.