AAP has released "Publisher Revenues Up 1.6% to $1.05 Billion in May."
Here's an excerpt:
Professional Publishing was down 28.5% in May 2016 vs. May 2015. These categories include business, medical, law, scientific and technical books. University presses were up 4.4%. Year to date, professional books are down year-to-date, and university presses are flat.