The presidential campaign of 2008 was one of the most dramatic in decades. The best-seller Game Change took readers behind the scenes with Hillary Clinton, Sarah Palin and the candidates of both parties. It was turned into an HBO movie. Now the same authors have followed up with Double Down, an account of last year's campaign. Mark Halperin of Time magazine is a co-author along with John Heilemann of New York magazine and MSNBC.