Interesting article and interactive chart from The Economist (clear year numbers to move the chart along). By their projection, eBook sales will exceed printed book sales in the US in 2018. Comments to the article point out that the data may underrepresent eBook sales by relying on ISBNs.
Whether or not that's true, and given other data that suggests the penetration of eBooks varies by category and country, the trend is clear -- as it has been for several years.
Amazon is the leader in eBook sales in the US, and, apparently, Barnes & Noble and Apple switch back and forth from month to month as to who is second and third.