Latest News: The Association of American Publishers (AAP) has released their estimate of 2006 book sales. They indicate that U.S. publishers earned $24.2 billion, which was down .3% over 2005's $24.26 billion in net sales.
The data isn't perfect. Only 81 publishers' data is included. Publishers Lunch points to "flaws and incompleteness." Shelf Awareness gave an example of incompleteness when they reported: "for religious books and e-books, data includes sales information from the Evangelical Christian Publishers Association and the International Digital Publishing Forum respectively."
According to the Los Angeles Times: "Religious book sales fell 10.2% to $745 million while audio-books dropped 11.7% to $182 million. Spurred by progress in digital technology, sales of e-books jumped 24.1% to $54 million."