“Go Set A Watchman,” a follow-up to Harper Lee’s “To Kill A Mockingbird,” will be released Tuesday. (AP Photo/Harper) By Hillel Italie AP National Writer NEW YORK – Harper Lee’s attorney has provided her fullest accounting yet of how she came upon the manuscript for “Go Set a Watchman,” which comes out Tuesday. She is also raising the possibility of a third book by the author of “To Kill a Mockingbird,” a suggestion challenged by a leading biographer. In an op-ed piece posted late Sunday on the Wall Street Journal’s web site, Tonja Carter wrote that last summer she was at a gathering of Lee’s friends and family members when “talk turned” to a possible second novel by the author of “To Kill a Mockingbird.” According to Carter, it was the first time she had heard of the book’s existence.