Emeritus is right - but it really doesn't matter because, as has been pointed out already, the books are not related. They are 4 totally seperate stories.
from what I remember of them I preferred Angels and Demons to the Da Vinci Code (but they are actually very similar), and hated Deception Point. Digital Fortress is the best of the lot.
There are some editions of DaVinci Code with the artwork included. I read it without the art and kept trying to look them up for reference. If you can get one withthe art it will be more interesting.