By Aimee Caruso Valley News Staff Writer Charlestown — A new 561-page book about Charlestown’s history will be available starting Saturday. Third History of Charlestown, New Hampshire focuses on the period spanning 1950 to 2009. The town’s Heritage Commission came up with the idea for the black-and-white book several years ago, said Joyce Higgins, who is the commission’s chairwoman and a member of the Charlestown Historical Society. Last year, voters …