"Easily half of the NYRB writings are about non-fiction" means that easily half of the books reviewed are non-fiction books. And, looking over the books listed

"Easily half of the NYRB writings are about non-fiction" means that easily half of the books reviewed are non-fiction books. And, looking over the books listed in the current issue (http://www.nybooks.com/issues/2014/oct/23/ http://www.nybooks.com/issues/2014/oct/23/), there certainly seems to be a solid majority of non-fiction books.