ABOARD A U.S. MILITARY AIRCRAFT — Before taking the highest office in the Pentagon, Defense Secretary James Mattis was already a prolific reader, boasting a library of more than 7,000 books and frequently revisiting a handful of titles for reference.
Despite his increased responsibilities and demanding schedule, Mattis' extraordinary reading habits haven't slowed.
He's currently reading "Earning the Rockies: How Geography Shapes America's Role in the World" by Robert D. Kaplan and "Partners in Command: George Marshall and Dwight Eisenhower in War and Peace" by Mark Perry, chief Pentagon spokeswoman Dana White told CNBC as the secretary wrapped up a five-day trip around the world.
Mattis, a revered Marine with a military career spanning four decades, credits his leadership success to his voracious reading habits.
"Thanks to my reading, I have never been caught flat-footed by any situation, never at a loss for how any problem has been addressed before. It doesn't give me all the answers, but it lights what is often a dark path ahead," Mattis wrote in a 2003 email to military historian Jill Russell.
Hailed for his battlefield prowess and kinship with rank-and-file servicemembers, Mattis explained that the best way to hone war-fighting skills is to leverage lessons learned from history.
"A real understanding of history means that we face NOTHING new under the sun," Mattis wrote. "We have been fighting on this planet for 5000 years and we should take advantage of their experience. Winging it and filling body bags as we sort out what works reminds us of the moral dictates and the cost of in competence in our profession."
Before Mattis became President Donald Trump's Defense secretary, the four-star Marine Corps general led the U.S. Central Command, the combat command responsible for the wars in Iraq and Afghanistan.
Throughout his military career, Mattis was affectionately referred to as "Mad Dog" and "warrior monk." He was known for his strategy, matter-of-factness and disdain for PowerPoint, which is recognized as the U.S. military's signature teaching tool.
Instead, Mattis chooses to arm himself with books.
In October 2016, a Marine asked Mattis how he continues to develop as a leader. Mattis once again pointed to reading.
"You stay teachable most by reading books, by reading what other people went through," Mattis said. "I can't tell you the number of times I looked down at what was going on on the ground or I was engaged in a fight somewhere and I knew within a couple of minutes how I was going to screw up the enemy. And I knew it because I'd done so much reading."
He added: "I knew what I was going to do because I'd seen other similar situations in the reading. I knew how they'd been dealt with successfully or unsuccessfully."
In an interview with author R. Manning Ancell for the 2017 book "The Leader's Bookshelf," Mattis shared some of his favorite titles for both the tactical and strategic reader:
"I guess on a tactical level, there was a novel by M.M. Kaye called 'The Far Pavilions,' and, of course, Guy Sajer's 'The Forgotten Soldier,'" Mattis told Ancell. At the strategic level, "you can't go wrong when you read [Ulysses S.] Grant's 'Memoirs' or Viscount Slim's 'Defeat into Victory.'"
Mattis' reading extends well beyond just military history. His collection includes memoirs of those who served at the highest levels of government, including recent officials, and histories of some of America's founding fathers.
"I was executive secretary for two secretaries of Defense, I worked closely with three others and when you read [former Defense Secretary Robert] Gates' book 'Duty,' you get a real sense of the breadth and the gravity of what faces people at that level," Mattis said.
"And in some way you look back on Will and Ariel Durant's 'The Lessons of History' or Ron Chernow's book on Alexander Hamilton, and you realize, man, you can get an awful lot out of people who have been through this sort of thing and studied the ones who did it before," he said. "Then you realize how few things are really new under the sun if you do good reading."
Mattis' deep attachment to his books was painfully clear when he became secretary of Defense last year.
"He told me just before he took over at the Defense Department that one of the hardest things he has ever done in his life was to go through his books and give them away," Ancell told CNBC.
Ancell, who co-authored "The Leader's Bookshelf" with former Supreme Allied commander Adm. James Stavridis, said Mattis had a personal collection of 7,000 books before retiring from the military.
"For most of his career he had packed all of them up at the end of one assignment and had them shipped to his new assignment," Ancell said of Mattis' book collection. "And when he retired out in California he discovered he just didn't have the room anymore."
"I knew I wouldn't read them again," Mattis explained to Ancell, adding that he kept his books on geology, military history and the American West.
While Mattis is one of the most prolific readers to ever hold the Pentagon's highest office, Ancell says "he does have a competitor."
The author said that Mattis has frequently deferred to another decorated Marine Corps general as being even more well-read than he is: John Kelly.
Kelly, who is Trump's White House chief of staff, is "the only man I have ever known who reads more than I do," the Defense secretary has said.
Kelly, a former commander of U.S. Southern Command, was known to begin his day at 3 a.m. so that he could read for two hours before his morning workout routine. He was such a prolific reader that he was tapped to compile the first Marine Corps commandant's reading list.
The list that Kelly helped create started an annual tradition. It has since sparked other service branches and intelligence agencies to create their own reading lists.
"If there's not a war going on, the only way to learn about war is to read about it, and to read about how other people did war," Kelly told Ancell for "The Leader's Bookshelf."
Like Mattis, Kelly is an advocate of professional development through reading.
"I'll have people contact me a lot to say, 'You know, sir, I was just thinking, can you give me an idea of a book to read on military intelligence or something like that?' I would say, sure; these are four or five really good books on that," Kelly said for the book.