Mr. Wolff characterized the book as an investigation of what it was like to work with Mr. Trump. He wrote that the president’s associates called him a “moron” and an “idiot,” and almost unanimously described him as being “like a child.”

“What they mean by that is he has a need for immediate gratification,” Mr. Wolff said. “It’s all about him.”

On Thursday, a lawyer for the president sent an 11-page letter to the book’s publisher, Henry Holt and Co., saying that it included false statements about the president that “give rise to claims of libel.”

In reaction, the publisher moved up the release date. Originally scheduled for a Tuesday debut, “Fire and Fury” was made available early Friday morning.

“We see ‘Fire and Fury’ as an extraordinary contribution to our national discourse, and are proceeding with the publication of the book,” the publisher said in a statement.

The White House has characterized the book as a “complete fantasy” full of “tabloid gossip,” but it is not only the administration that has questioned Mr. Wolff’s reporting. Some journalists have also expressed skepticism and pointed to past criticism of Mr. Wolff’s work. In 2004, The New Republic said the scenes in his columns “aren’t recreated so much as created — springing from Wolff’s imagination rather than from actual knowledge of events.”