Conspiracy Theory is a book that appears in Grand Theft Auto: San Andreas.
It can be seen being read by Mike Toreno, a corrupt federal agent, during the opening cutscene of the mission Verdant Meadows. It is unknown who wrote the book.
Little is known about the book, though Toreno tells Carl that it claims Adolf Hitler killed himself and that the United States nuked Japan. Funnily enough, these events did happen in real life, and the book may be just a joke that Toreno is part of a conspiracy himself.