Net sales of e-books jumped to 15 percent of the market in 2011 from 6 percent in 2010, according to a report by the Association of American Publishers and the Book Industry Study Group. The groups compiled data provided by nearly 2,000 publishers.

Total overall U.S. book market sales declined 2.5 percent to $27.2 billion in 2011 from $27.9 billion in 2010, the report said.

While ebooks increased in strength, bringing in more than $2 billion in 2011, the majority of publishers' revenue still came from print books, with $11.1 billion in 2011.

"We're delighted to see it (the report) affirm that the industry has remained steady, and has even grown in some areas, in what continues to be a challenging economic time and through such significant transformation," said Len Vlahos, executive director of the Book Industry Study Group, in an email.