That's a nearly 48x increase in e-book sales in just 4 years, with hardcover and paperback sales remaining relatively level. 

Despite a recession (and thanks to 50 Shades of Gray) net revenue has actually increased by nearly 2 billion dollars.

No wonder publishers are shilling e-books so hard. That's the same amount of work, but with nearly double the profit margin.

Sources: Bookstats; Internal HarperCollins Presentation