Andrew Carroll believes that American war veterans must save their letters. Letters are history’s most personal source, for they’re meant to be read by a narrow audience and lack the normal pretense of official reports, newspaper clippings and magazine articles. At the urging of many American veterans, Carroll, editor of the book “War Letters” and founder of the Legacy Project, has compiled a wonderful new collection of correspondence from around the world. “Between the Lines” (Scribner, 512 pages) includes the letters and e-mails of American men and women, allies, enemies and civilians, written from the time of the French and Indian War to the current actions in Afghanistan and Iraq.