The No. 1 nonfiction book in 2003 was Pastor Rick Warren's "The Purpose-Driven Life," according to the Book Industry Study Group. The self-help guide has sold 17 million copies.
Top sellers on the fiction side include "The Da Vinci Code," "The Five People You Meet in Heaven," and "Armageddon," all titles with a spiritual bent.
"What we are looking at is unquestionably religious books becoming mainstream," said Albert N. Greco, the study group's statistician and a professor at Fordham University. "They are so hot that big chains like Barnes & Noble and mass merchandisers such as Wal-Mart are allocating more space."