"You stay teachable most by reading books by reading what other people went through," Mattis has said.

Secretary of Defense Jim Mattis arrives at the 149th annual Department of Defense National Memorial Day Observance at Arlington National Cemetery, May 29, 2017. Department of Defense photo

Defense Secretary James 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. Mattis, hailed for his battlefield prowess and kinship with rank-and-file soldiers, 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 incompetence 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 combatant 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.

Developing leadership

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 [Ulysses S.] Grant's 'Memoirs' or Viscount Slim's 'Defeat into Victory.'"

More than military history

U.S. Marine Corps Gen. James Mattis, Commander, U.S. Central Command, talks to Marines on Dec. 25, 2011, in Kabul, Afghanistan. Department of Defense photo

Mattis' reading extends well beyond just military history. His collection includes memoirs of those who served at the highest levels of government office, including recent officials, and histories of some of America's founding fathers. "I was executive secretary for two secretaries at 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."

A deeply personal library

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 U.S. Navy Adm. James Stavridis, said that 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.

A literary kindred spirit in the White House

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.

White House Chief of Staff John Kelly speaks during a daily news briefing at the James Brady Press Briefing Room of the White House. Alex Wong | Getty Images