A quote from Publishing Executive magazine:
June 4, 2014 By Denis Wilson
Consumer books electronic revenue will reach US$8.7 billion in 2018, up from US$4.5 billion in 2013, a CAGR of 14 percent. Digital revenue will overtake consumer books print/audio revenue in 2018 with growth in tablets encouraging digital consumption of books.
No surprises. But the trend appears to be accelerating. Advertising is shifting largely to mobile. It’s a massive shakeup.
One little surprise: they include audio in with print
So, does that mean digital delivery of audio is next? Or will the audio be built into our ePUB3 books?