Former U.S. Secretary of Defense Donald Rumsfeld will visit Fort Campbell, Kentucky to sign copies of his New York Times best-selling memoir, Known and Unknown, later this month.Rumsfeld served twice as defense secretary, first under President Gerald Ford and subsequently President George W. Bush. He has been visiting military bases around the country to promote the book, from which proceeds go to charities supporting those serving in the military and their families.The book signing will be at the Fort Campbell Base Exchange, May 27 at 2 p.m.