According to the Association of American Publishers (AAP) net sales revenue report for September 2011, the adult mass paperback category declined 54 percent (to $31 million) in September compared to the same period last year.

At the same time, adult hardcover sales dropped 18 percent (to $148.3) while eBook sales doubled to $80.3 million. Above, we’ve embedded the AAP’s sales chart.

Here’s more: “With September also being the start of the 2011-2012 academic year, Higher Education showed a nearly 9% gain in net revenue vs September 2010. Additionally, there was significant growth in the category of Professional Books (in the Professional and Scholarly Publishing division) in September, increasing nearly 13% … The September report represents data provided by 80 US publishers and is produced by the Association of American Publishers.”