Of course the book is fiction. And like most good fiction books it borrows some information from reality. These informations are gathered in an intelligent way but cannot be taken literally.
The grail is a very old mythos that has been described in libraries of literature (e.g. everything dealing with King Artus). However it is very unlikely that it exists in the form of the chalice that was used to collect Jesus' blood at the cross- and it is even more unlikely that it exists as a collection of secret documents proving a different role of women in the Christian church as postulated in the Da Vinci Code.
This does not make the book worse: in books it is allowed to invent whatever you want. The reason I didn't like the book too much are more in the structure of the story.
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