"I don't like him", the elder Bush told author Mark K. Updegrove in May 2016.
"I don't know much about him, but I know he's a blowhard. And I'm not too excited about him being a leader".
The book also revealed that George H.W. Bush doesn't like Trump and called him a "blowhard" when he was running for president.
CNN reached out to both former presidents' offices for further comments and they both confirmed that they had spoken with Updegrove on the record, but as a policy do not comment on books.
"And it wasn't just about Hillary Clinton becoming president, as the Republican Party was having a hard time finding itself".
This, despite standard etiquette among former presidents in which they generally refrain from speaking ill of one another.
"He's a blowhard", the 41st president tells historian Mark Updegrove in the book "The Last Republicans", which goes on sale November 14.
"I voted "None of the Above" for president, and Republican down ballot in 2016", he told CNN.
"I understand his frustration, because at the time there was the perception that Dick Cheney was the acting president", Updegrove told CNN. The two men, who served in the White House for a combined 20 years, revealed their assessment of the new commander-in-chief and the state of the Republican Party under his leadership in a new book.
That inspired the title of the book - which will be published November 14 by HarperCollins - as a quote that seemed to carry a double meaning.
George W. Bush's comments were slightly less inflammatory than his father's, although he told Updegrove that while he thought Trump could unify the country, it would require "humility", CNN reported.
Bush and George H.W. Bush speak to the crowd before game five of the 2017 World Series between the Houston Astros and the Los Angeles Dodgers at Minute Maid Park on October 29, 2017 in Houston, Texas. The Bushes felt stung by Trump's ground-burning attacks that helped destroy the campaign of Jeb Bush, the son and brother of the presidents.
"You can either exploit the anger, incite it, or you can come up with ideas to deal with it", the 43rd president said, explaining that Trump never had real solutions.