AAP has released "Ebook Sales Declined in 2015; Digital Audio Continued Growth."
Here's an excerpt:
Overall sales were up 0.8 percent to $7.2 billion compared to $7.1 billion in 2014.
Overall publisher revenue for 2015, however, was $15.4 billion, down 2.6 percent from the previous year. . . .
Ebook sales declined in 2015; digital audio continued to grow in popularity; trade publishers did better than educational and scholarly publishers; adult books performed better than other trade categories.