Robert Butterworth - The other 3 books were actually written before Da Vinci Code but no-one had heard of any of them until all the hype.
As to the original question The Eight by Katherine Neville, a mystery/puzzle/clues novel set in two time periods (French Revolution/'present day'). It revolves around a chess set that once belonged to Charlemagne and it was created in Bagdad. It involves Islam and oil. Oh yes! and it was first published in the 1980's, and was no doubt an inspiration for books like Da Vinci Code and Labyrinth.