Bock, Darrell L., Breaking the "Da Vinci Code": Answers to the Questions Everybody's Asking, Nelson Books, 2004.

This guide focuses on the three centuries following the birth of Christ to examine the suggestions the novel makes about early Christian history. It largely discredits the theories Brown puts forth in the novel.

Burstein, Dan, Secrets of the Code: The Unauthorized Guide to the Mysteries Behind "The Da Vinci Code," CDS Books, 2004.

In a study almost as long as the novel itself, Burstein collects interviews and essays from historians, scientists, archeologists, and theologians, some of whom have contrasting views about the questions the novel raises. This is considered one of the most comprehensive guides to the topics the novel engages, such as what is known about Jesus, Mary Magdalene, the Gnostic gospels, and secret societies.

Ehrman, Bart D, Truth and Fiction in the "Da Vinci Code": A Historian Reveals...