This article, by a publishing industry consultant, contains some interesting analysis.
http://www.idealog.com/blog/stats-ar...-our-business/
He thinks that 50% of fiction sales are ebooks, and around 25-35% are online sales of print books, leaving around 30-35% for print books sold in physical stores. I agree that this probably explains the decline in shelf space, especially as nonfiction has also declined in physical store sales, although much less.