"I don't like him. 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," Bush senior said in May 2016 in conversation with the book's author, historian Mark Updegrove.
The book, which focuses on the father and son and is called "The Last Republicans," also reveals that George W. Bush did not think that Trump would win the presidential race.
When Trump entered the race, George W.'s reaction was "interesting, won't last," he told Updegrove, who had several on the record discussions with the pair. But the former president explained that "when you're not out there and you're not with the people, you don't get a good sense of (the mood)."
George W. was unimpressed by Trump's claim that he doesn't need advisers. During the presidential campaign, Trump said "my primary consultant is myself and I have a good instinct for this stuff," when asked who his advisers were on foreign policy.
"Wow, this guy doesn't know what it means to be president," was the former President's reaction, Updegrove told CNN on Saturday.
George W., whose brother Jeb had run for the Republican nomination, said that he voted for Democratic candidate Hillary Clinton in the presidential election. George H. W. revealed that he voted "None of the Above."
The White House has responded to the revelations with criticism of Trump's predecessors' legacies.
"If one presidential candidate can disassemble a political party, it speaks volumes about how strong a legacy its past two presidents really had. And that begins with the Iraq war, one of the greatest foreign policy mistakes in American history," a White House official told CNN.