Trump 'could shift so quickly from topic to topic' on campaign trail, writes Gingrich in new book Trump "could shift so quickly from topic to topic," writes the former speaker.

 -- In Newt Gingrich's upcoming book "Understanding Trump," the former speaker of the House chronicles his time on the campaign trail with then-candidate Donald Trump, and explains to readers why he believes the real estate tycoon-turned-president won the election.

"President Trump is one of the most remarkable individuals to ever occupy the White House," Gingrich, 73, writes in the introduction of his book, which will be released on Tuesday. "His set of practical business experiences -- and his lack of traditional political-governmental experiences -- make him a unique president."

And the Trump family appears to be fond of Gingrich, as well. Eric Trump writes in the book's foreword that shortly after his father announced his plans to run for president, "it was clear that Newt Gingrich and his wife, Callista, were true friends of the Trump family."

Eric continues, "Newt became more than just a surrogate; he became a friend who profoundly understood my father's tenacity and his passion for one singular goal: to Make America Great Again!"

In "Understanding Trump," Gingrich expresses how he was in awe of how Trump "would shift with amazing speed" on the campaign trail.

"He could be flying to a rally with more than twenty thousand people, suddenly see a fact or a story and get Stephen Miller, an unsung hero of the Trump campaign, to add it to the speech as they moved toward the stadium," explains Gingrich, who was speaker from 1995 to 1999, in the book's first chapter.

"One of the keys to the campaign was Miller sitting at a table about ten feet behind Trump and banging away on the next speech or press release as they flew around the country," he continues. "Understanding Trump requires understanding the principles by which he could shift so quickly from topic to topic."

On a lighter note, Gingrich also writes about the president's fondness for fast food -- which comes as little surprise, considering he has posted to social media in recent months photos of himself eating McDonald's and KFC.

"Anytime a meal was served when I flew with candidate Trump aboard his nicely outfitted 757, it was invariably McDonald's, Wendy's, or similar fast food," Gingrich writes. "Here was this billionaire with a big plane and a professional crew, and his personal taste leaned main street American fast food."

He adds, "Friends who saw him in Palm Beach at the fancy Sunday brunch at his golf course reported the same pattern. Trump would wander through the line and get a cheeseburger and fries. It was a very practical reminder that in his heart Trump was raised as a middle-class guy from Queens -- not a Manhattan socialite."