The Pulitzer Prize-nominated book that served as the basis for the Oscar-winning movie starring Jack Lemmon and Sissy Spacek.   Charles Horman was an American freelance journalist and documentary filmmaker who had traveled to Chile in the early 1970s to explore a country that was undergoing significant changes under the then-Marxist President Salvador Allende. In the course of his research, Horman seems to have uncovered information about CIA involvement in a plot to overthrow Allende. In fact, the coup did take place with General Augusto Pinochet taking over as dictator then ordering the mass arrest of thousands of dissidents and opponents. Horman was one of thousands of people who was dragged from his home and never again seen alive. The American Embassy refused any assistance.   It seems that Horman was murdered by Chilean security police, although this was never publicly acknowledged. Horman's father, Ed, a patriotic American businessman, traveled to Santiago where officials of the American Embassy, led by the ambassador himself, offered to help him search for his son--but these same embassy officials knew that Horman was dead.   Published in 1978, five years after Pinochet took over Chile, Missing is a harrowing tale. It is an explosive story that touches on political matters that are still relevant today. Hauser calmly sets about laying the groundwork for his story, examining both the facts as well as the more mysterious elements of this true story.