The Da Vinci Code is a lot of fun. The premise is typically conspiratorial: that throughout the centuries secret organizations have conspired to prevent the truth of Jesus's geneaology from being revealed and the role of women in early Christianity was very different from what evolved.
Aside from the obvious truth that humans can't keep any kind of secret, conspiracy theories make for entertaining "what-if" stories. It's important, however to maintain a rational base and several books have been written debunking claims made in The Da Vinci Code. The right-wing Christians, typically paranoid, were the first to get all upset and emails began circulating all over the web decrying his book. The author, Dan Brown, fed the flames by asserting the historicity of the art and details in the introduction. That just makes it more fun, of course, except for the illiterate idiots who went nuts.
Much of Brown's "historical" information seems to be drawn from an eyebrow raiser of several years ago, Holy Blood, Holy Grail" by Baigent -- I can already hear you moaning, "not another Holy Grail book" -- that purports to "document" the existence of the Priory of Scion and the marriage of Mary Magdalene and Jesus.
For a good historical review, I recommend, Bart Ehrman, Chairman of the Department of Religious Studies at the University of North Carolina at Chapel Hill, who is the author of Truth and Fiction in the Da Vinci Code, as well as Lost Christianities and the Battles over Authentication and Lost Christianities: The Battles for Scripture and the Faiths We Never Knew. (The Teaching Company offers several of Ehrman's lectures on CD, check your local library, or click here) His books and lectures provide very accessible accounts of the early Christian struggle to define dogma and doctrine. Another very interesting book is When Jesus Became God that discusses the battle between the Arians and the Athanasians, two groups with a very different view of Jesus's divinity and the role Constantine played in shaping the outcome. (For a Christian philisopher's view of the negative impact of the alliance of the state and Christian orthodoxy see The Subversion of Christianity by Jacques Ellul. The effect of that orthodoxy and the development of heresy and blasphemy, Leonard Levy's Blasphemy: Verbal Offense Against the Sacred, from Moses to Salman Rushdie should not be missed. An excellent book.
Additional reading recommendations: Elaine Pagels' books including The Gnostic Gospels and Beyond Belief : The Secret Gospel of Thomas.
If nothing else, perhaps Brown will have encouraged people to read more about how the Gospels were defined and doctrines codified. It's a fascinating story. The truth can often be more unsettling than fiction. As Ehrman says in the epilogue to Truth and Fiction:
"And for some of us the historical record really does matter, possibly because in some ways, history is like any other good story. It is a narrative that we tell and retell, filled with characters that we can relate to, with plots and subplots that we somehow feel apart of. The past is a story that we ourselves can live in, one that can inform our lives in the present. It is a true story, one that contributes to our sense of ourselves and our place in the world."