This is a very readable book. Its strength is in its discussion of Gnosticism then and now.
Ehrman, Bart D., 2004, Truth and Fiction in The Da Vinci Code: A historian reveals what we really know about Jesus, Mary Magdalene, and Constantine, Oxford Universtiy Press, New York.
Written by a historian and not from a Christian perspective this is a great book if you'd like more information on the sources of the New Testament.
Olson, Carl E. & Miesel, Sandra 2004, The Da Vinci Hoax, Ignatius Press, San Fransisco.
This book is the most scholarly that I read and has lots of footnotes for further research. The authors are Catholic and therefore pick up some of the more Catholic claims. They also cover some of the artistic errors and provide a detailed history of the Grail myth.
Witherington III, Ben 2004, The Gospel Code: Novel claims about Jesus, Mary Magdalene and Da Vinci, InterVarsity Press, Downers Grove, Illinois.
Another evangelical Christian, Witherington's book is easy to read and covers most of what Ehrman and Bock to but in a slightly different way.
Other sources used
Wegner, Paul D. 1999, The Journey from Texts to Translations: The origins and development of the Bible, Baker Book House, Grand Rapids, Michigan.