E-books have become a “normal means of consuming content,” but the rapid growth of the format’s early years are over, that was one of the primary takeaways from the final installment of Consumer Attitudes Toward E-Book Reading, the four-year project conducted by the Book Industry Study Group that traced the evolving role of e-books in the trade market.

According to the survey, at the end of the second quarter of 2013, e-books accounted for just fewer than 30% of units and approximately 14% of sales, figures about equal to the fourth quarter of 2012. While both e-book sales and unit have risen considerably since the first quarter of 2011, their share of the market dipped slightly between the first quarter of 2013 and the second period of the year. Jo Henry, director of Nielsen Book Research, which conducted the project for BISG, observed: “It is clear from four annual research surveys that e-books are in the later stages of the innovation curve and have settled into reasonably predictable consumption patterns. The likelihood of future growth will, in part, depend on improving the value perception of e-books among less committed users.”

One of the “value adds” that customers seem the most interested in is e-book and print bundles, with 48% of respondents to the August survey saying they would be willing to pay more for bundles. Consumers also said they would be willing to pay more for e-books if the books could be either given away or re-sold.

The research also provided more evidence of a hybrid print-e-book market with over 30% of August respondents reporting they purchase e-books and print books interchangeably—the same percentage as in September 2010. Just over 40% of e-book buyers said they purchase mostly e-book and fewer print books, a percentage that peaked at just fewer than 50% in February 2013.

One thing that hasn’t changed since the study was launched is the dominance of Amazon in the e-book market. Sixty-seven percent of e-book buyers said that their primary source for e-books was Amazon, with 51% saying they buy most of their e-books from the Amazon site and another 16% using an Amazon app. Barnes & Noble held onto second place with a 12% share divided almost equally between their Web site and the B&N app. Apple was third with an 8% share.

The full report is available from the bisg.org site.