Last August a hotel maid in Basra who had popped in to clean a room found the dead body of guest who had been staying there for almost a week. The dead man was Steven Vincent, a free-lance American journalist who was doing research for a book on the situation in southern Iraq. When the police examined the body and the scene it concluded that Vincent had been murdered. But who killed Vincent? A few days before his tragic death, the journalist had published an op-ed column in The New York Times charging that Iraqi Shiite parties had infiltrated the...