Atlanta’s most celebrated author, Margaret Mitchell, had a life before she wrote “Gone With the Wind,” and her proud hometown is about to celebrate that fact. A new museum exhibit devoted to Mitchell’s days as a journalist is set to open tomorrow, which would have been the late author’s 100th birthday. (She was killed in 1949, 10 years after the release of the blockbuster movie version of her one and only novel, when she was struck by a speeding taxicab while crossing Atlanta’s Peachtree Street.) The Associated Press reports that a display at the Margaret Mitchell House & Museum will re-create Mitchell’s office at the Atlanta Journal and feature the actual desk where she worked, as well as her articles and photographs. In addition, two new books are scheduled for release this week: “Before Scarlett: Girlhood Writings of Margaret Mitchell” (containing stories she wrote as a youngster) and “Margaret Mitchell, Reporter” (featuring her writings for the Journal). Margaret Mitchell, by the way, was her pen name. She was known as Peggy Marsh.