The investigation surrounding the crash of TWA Flight 800 last July has not yet wrapped up, but Zebra Books is rush-releasing The Downing of TWA Flight 800, which attributes the cause to the now-infamous "missile theory," with true believers (often chatting on the Web) claiming there's proof (a mysterious reddish-orange residue and FAA radar report) that a U.S. missile shot down the plane. The mass market paperback, which will be published in mid-April with a 500,000-copy first printing, was written by former Los Angeles police officer James Sanders, who somehow had "access to physical evidence from the wreckage," according to Zebra Books publicist Diane Wright. As an investigator of automobile crash scenes, Sanders has co-written two books in the early 1990s on American POWs in the Vietnam War and the Soviet Union. And what about a book by Pierre Salinger, a surprising, vocal proponent of the missile theory? At press conferences last week, the former Kennedy press secretary, U.S. Senator and ABC News Paris bureau chief hinted at doing a book on airplane crashes, but at press time agent Ed Victor had not confirmed any deal. Salinger's next confirmed book will be a JFK photo book for Penguin Studio this July.