The Manhattan District Attorney’s Office is looking into the mysterious death 51 years ago of newspaper writer and “What’s My Line?” star Dorothy Kilgallen, who was investigating the JFK assassination, The Post has learned. The stunning development comes after a new book, “The Reporter Who Knew Too Much,” suggests Kilgallen was murdered to shut down her relentless pursuit of a Mafia don linked to JFK and Lee Harvey Oswald. Joan Vollero, a spokeswoman for DA Cyrus Vance Jr., confirmed that a staffer has read the book, and reviewed a letter from author Mark Shaw citing new leads, medical evidence, and...