The Da Vinci Code
According to Sharan Newman in The Real History behind the Da Vinci Code, Dan Brown based The Da Vinci Code on a book called Holy Blood, Holy Grail by Baigent and Leigh. This book was based on some papers found in the National Library of France, and which were forgeries. The papers were written to show that the man who wrote them was really the true king of France and head of an ancient secret society.