Dan Brown, whose “Da Vinci Code” has become the bestselling hardback adult novel of all time, has been accused of stealing his plot for the book from a non-fiction work called The Holy Blood and the Holy Grail. If they win, the plaintiffs “will seek an injunction preventing further infringement of their copyright. In theory, this could bar Random House from publishing Brown’s book, which has sold more than 40 million copies, and even threaten the British release of the £53m film adaptation, starring Tom Hanks.”