Dan Brown has finally delivered the long-awaited follow-up to his global smash “The Da Vinci Code.” It’s called “The Lost Symbol” and will be published Sept. 15 by Doubleday. First print run: five million copies. “The Lost Symbol,” which was originally scheduled for a 2005 delivery, will again feature Robert Langdon, the protagonist of “The Da Vinci Code,” which sold 81 million copies. The new novel takes place over the course of 12 hours and follows “Robert Langdon through a masterful and unexpected new landscape.” Brown, on his Web site, danbrown.com, said the work had Langdon “embroiled in a mystery on U.S. soil” and “explores the hidden history of our nation’s capital.” He also says that it is “set deep within the oldest fraternity in history,” the Masons.