Leavy, the former Washington Post writer who also has authored bestselling biographies of Sandy Koufax and Mickey Mantle, certainly isn’t the first to try to dig deeper into the life of one of the most intriguing characters in the history of sports. What sets “The Big Fella: Babe Ruth and the World He Created” apart from earlier attempts to identify the true essence of the man is an unprecedented look back into Ruth’s long-neglected childhood and a magnified focus on how his tremendous popularity helped birth the cult of personality in America.