Despite the slump to $724 million in total revenues, religious books "still performed well" with a compound growth of 4.5% over the same time period-noted the report released earlier this week, which provides annual total book sales.

The AAP estimates were drawn from data from the Bureau of the Census as well as sales information from 81 publishers "inclusive of all major book publishing media market holders," the association said.

The religion sales drop was topped by audiobooks, which fell 21% to $172 million, while sales fell 12.4% in children's hardcover and 13% in adult hardcover. Mass-market paperbacks decreased 3%, while book clubs and mail-order fell for the sixth year to $600 million, a 3.4% drop in sales.

The biggest growth occurred in e-book sales, which shot up 68.4% to $113 million, while children's and adult paperback sales grew 6.4% and 3.6%, respectively.