The millionaire author was on the stand for the second day, defending his work against a copyright infringement suit brought by Michael Baigent and Richard Leigh, authors of a 1982 nonfiction book, "The Holy Blood and the Holy Grail."
The suit is not against Brown, but his publisher Random House, which also published "The Holy Blood and the Holy Grail."
"The Da Vinci Code" is still a bestseller, and "The Holy Blood and the Holy Grail" has seen sales soar 1,000 percent. Even attendance on a London walking tour of "Code"-related sites has been up.
One walker expressed some cynicism to MacVicar, saying, "The only winner in this is the publishing company. They're the ones going to be selling books like crazy."
Much of Tuesday morning's testimony was taken up with questions about markings and underlinings in Brown's copy of "The Holy Blood and the Holy Grail."
In the months after the publication of "The Da Vinci Code," which has sold more than 40 million copies, Brown said he had to go back to his research to answer questions posed by readers while on tour.
Among the books he consulted was "The Holy Blood and the Holy Grail."
"I am a novelist, not a historian," Brown said. "I needed to go back, so I could defend the work."
Brown, who had published two earlier novels, said he was nearly overwhelmed by response to the religious thriller.
"What happened when 'The Da Vinci Code' came out was totally foreign to me," Brown said.
"When I published 'Angels and Demons' I would go to bookstores and give talks, and there would be five or six people in the audience — and three people were bookstore owners who had taken off their badges so I wouldn't feel bad.
"When 'The Da Vinci Code' came out, I was suddenly talking to 300 or 400 people."
On Monday, Brown said he was shocked at the copyright infringement claim.