I thought these numbers were interesting:

Total sales by format

Downloaded audio: $552 million
eBooks: $2.84 billion
Paperback books: $5.23 billion
Hardback books: $5.40 billion

Number of units sold by format

Physical Audio & Downloaded Audio: 81 million
eBook: 424 million
Paperback & Mass Market: 1.18 billion
Hardback: 577 million

From:

U.S. Publishing Industry’s Annual Survey Reveals Nearly $28 Billion in Revenue in 2015