The book alleges that many White House insiders consider the US President to be "a moron"
US President Donald Trump has accused the author of a highly critical new book of being a "fraud" and slammed the work as "fiction."
The President's fitness for office has been questioned since the publication of Fire and Fury, which claims his former chief strategist, Steve Bannon, questioned his competence.
The book's author, Michael Wolff, has also alleged that many people close to the President describe him as a "moron, an idiot" and that Mr Trump's deputy chief of staff said working with him is like "trying to figure out what a child wants."
Speaking at Camp David, President Trump attacked Mr Wolff's version of events.
"I consider it a work of fiction and I think it's a disgrace that somebody is able to have something, do something like that," he told reporters.
"The libel laws are very weak in this country. If they were strong, it would be very helpful.
“You wouldn't have things like that happen where you can say whatever comes to your head.
"He said he interviewed me for three hours in the White House - it didn't exist. It's in his imagination."
But the President did concede that he did a "quick interview with him a long time ago."
"I don't know this man," he added. "I guess sloppy Steve (Bannon) brought him into the White House quite a bit... that's why sloppy Steve is looking for a job."
Earlier, Mr Trump also responded to Wolff's claim that "one hundred per cent" of the people who work with the President question his mental fitness for office.
Mr Trump tweeted: "Throughout my life, my two greatest assets have been mental stability and being, like, really smart.
"I went from very successful businessman, to top TV star to President of the United States (on my first try).
"I think that would qualify as not smart, but genius - and a very stable genius at that."
In a previous Twitter tirade, Mr Trump dismissed the book - said to be based on more than 200 interviews - as being "full of lies, misrepresentations and sources that don't exist."
"Michael Wolff is a total loser who made up stories in order to sell this really boring and untruthful book," he wrote.
"He used sloppy Steve Bannon, who cried when he got fired and begged for his job. Now sloppy Steve has been dumped like a dog by almost everyone. Too bad"
Speaking in defence of his exposé, Mr Wolff said he spent three hours with Mr Trump and "spoke to people who spoke to the President on a daily, sometimes minute-by-minute basis."
The writer has claimed his revelations will eventually "bring down" Mr Trump.
"One of the interesting effects of the book so far is a very clear emperor-has-no-clothes effect," he told BBC Radio 4.
"The story that I have told seems to present this presidency in such a way that it says he can't do his job."
"Suddenly everywhere people are going, 'Oh my God, it's true, he has no clothes.'
"That's the background to the perception and the understanding that will finally end [...] this presidency."