The Book Spoiler doesn’t describe many book endings yet (two, to be exact), but it’s a neat idea. is more comprehensive though still limited:
We’re asking bestselling authors to talk with us about the endings of their books, and they are generously sharing their thoughts and comments. In candid remarks available nowhere else — on the Web or in traditional media — these authors reveal how they came up with those satisfying conclusions.
For The Da Vinci Code:
The secret? That Jesus Christ not only was married to Mary Magdalene, but that they produced a child. The clues that Langdon [protagonist] follows eventually leads him to find the resting place for the remains of Mary Magdalene, under an inverted Pyramid of glass that resides in front of the Louvre.
(via Blog of a Bookslut)