A writer working on a biography of Harper Lee came to my high school 6 or 7 years ago to give a presentation about her. He told us that she had written another book but didn't want to publish it due to the pressure she felt from the success of To Kill a Mockingbird, so she planned on having it published after she died. This is probably the book he was talking about.
I can't remember exactly who the writer was, but he spoke about his experience interviewing Kurt Vonnegut for his biography, so it was probably Charles Shields.
Apparently, they only just found this manuscript hidden in a box "in the fall of 2014", so it was probably not the new book that person was talking about. In fact, this was the original book before "To Kill a Mockingbird" was written.
I can't remember exactly who the writer was, but he spoke about his experience interviewing Kurt Vonnegut for his biography, so it was probably Charles Shields.