In June of 2011 Apple announced that 130 million ebooks were sold through iTunes. In October of 2012 it announced that 400 million sold.
That means 270 million ebooks were sold in 16 months. Or about 17 million units per month, on average. It also suggests 2012 ebook sales of about 200 million units. The following graph shows the download rate of books relative to apps and songs: