US President Donald Trump dismissed journalist Bob Woodward's bombshell book as a “joke” on Monday, a day before readers get their first full look at his detailed account of a White House in disarray.

“Fear: Trump in the White House” hits US bookstores on Tuesday after a weeklong buildup, with published excerpts, leaks and interviews that portray Trump as dangerously erratic and uninformed.

On the eve of its release, the president took to Twitter with a preemptive round of attacks, denouncing as fiction Woodward's reconstruction of scenes in which frustrated aides scramble to contain the mercurial president's reckless impulses.

“The Woodward book is a Joke - just another assault against me, in a barrage of assaults, using now disproven unnamed and anonymous sources,” Trump said in the first of a series of tweets.

“Many have already come forward to say the quotes by them, like the book, are fiction. Dems can't stand losing. I'll write the real book!”

Trump is quoted in the book brutally insulting members of his own Cabinet, referring to Attorney General Jeff Sessions as “mentally retarded” and former chief of staff Reince Priebus as “a little rat.”

US Defense Secretary Jim Mattis, in turn, is quoted as saying the president has the mental capacity of a “fifth or sixth grader,” while current chief of staff John Kelly is quoted as calling Trump “an idiot.”

“We are in Crazytown,” Kelly is quoted as telling a small group of confidants. “I don't even know why any of us are here. This is the worst job I've ever had.”

Mattis, Kelly and other senior aides have denied making the remarks.

“I don't talk the way I am quoted. If I did I would not have been elected President. These quotes were made up,” Trump said last week, calling the book a “scam. “