The profitability and sales of Michael Wolff's scathing book about United States President Donald Trump appear to have come under attack by WikiLeaks, which has posted a link to a free online copy.

Fire and Fury has become an instant best-seller since it went on sale on Friday, with hard copies sold out in many stores within an hour.

Mr Trump has begun legal action against the author, seeking to discredit the book's allegations against him as fake and disputing Mr Wolff's claim that he personally interviewed the President before writing the book.

Now, in an unprecedented move, Wikileaks has posted a link on Twitter to a Google Drive document that appears to contain the entire manuscript, although it is unclear whether it is the final version that went to publication.

WikiLeaks has given no explanation for its move.

It is unclear whether it was seeking to undermine the book's sales or simply provide an alternative copy of the book, since it sold out.

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But it is the second time WikiLeaks has apparently supported the US President.

During the 2016 presidential campaign, the whistleblowing site published a substantial number of embarrassing emails from Hillary Clinton that undermined her campaign for the presidency.

Meanwhile, Mr Trump's own tweets appear to have contributed to boosting the book's sales, at least according to some customers who bought it.

"I hate paying retail price for anything, but I made an exception buying this book because Trump wanted to stop its publication," wrote one reviewer on Amazon.com.

"In essence, I bought the book precisely to spite Trump. Hopefully the author will donate a portion of the proceeds to the movement to impeach Trump."

"Good Read. Probably wouldn't have ever read thanks for the book suggestion Donald," wrote another.

Eighty-five per cent of reviewers have given the book a 5-star rating, although US media have reported that Amazon was deleting many negative reviews.

Many buyers are still scathing of the book.

"Not worth wasting your time reading this nonsense. Instead should of bought a box of chocolates," wrote one.

"I found the writing difficult. There are too many grammatical errors and run-on sentences. Have your dictionary ready. He likes to show how smart he is by using pretentious words. I hoped this would be a fun read but after chapter one, I returned it for a refund," said another.

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The book's huge publicity also appears to have inadvertently boosted sales of at least one other book with the same title, including the 2008 book Fire and Fury: the Allied Bombing of Germany, 1942-1945.

Its author Toronto University professor Randall Hansen was grateful for the boost in sales.

Berlin-based journalist Markus Wierz joked: "I wonder how many pages people will read until they get that they are reading the 'wrong' book."

Another journalist Euan McKirdy at CNN tweeted: "Really hope that T.J. Coles, author of "Fire and Fury: How the US Isolates North Korea, Encircles China and Risks Nuclear War in Asia" is getting some unexpected additional book sales this week."

The Amazon site offers at least four books called Fire and Fury including one on the "brink" of World War III, as well as Sharada M Subrahmanyam's Fire's Fury and James D Delk's Fires and Furies, about the Los Angeles riots of 1992.