E-book sales continue to grow in both the U.S. and U.K., but to varying degrees. According to the latest results from Nielsen Books & Consumer’s survey of America’s book-buying behavior, e-books accounted for 23% of unit sales in the first six months of 2014, led by the adult fiction and the young adult categories, both of which saw e-books take a 30% share of unit sales in the first half of 2014. While e-books have been a meaningful part of adult fiction since the format’s early days, there has been a notable surge in YA e-books in 2014.

Beginning in 2012 with the success of the Hunger Games trilogy, YA e-books have taken off, and that trend has continued in 2014, fueled by strong e-book sales of titles like the Divergent series and The Fault in Our Stars. E-book sales represented only 22% of unit sales in the adult nonfiction category, and 13% of children’s sales excluding young adult.

Meanwhile, in the U.K., the Nielsen data show e-books continuing to grow, but not enough to halt an overall volume decline in the book market, as print sales decline at a greater rate than the growth in digital. Notably, however, the value of the e-book U.K. market this year has increased more sharply than volume, with consumers beginning to pay higher prices for their digital content.

In early 2013, e-book prices were at a historic low, with U.K. consumers paying £3.39 on average for each e-book, about half of what they were paying for a paperback and a third of what they were paying for a hardcover title. By the first half of 2014, consumers were paying £4.14 on average for e-books, a 12% increase.

Although e-books remain much more important to the adult fiction market than to the other main categories, the latest data reveals that nonfiction and children’s e-books are at last breaking through in the U.K., with significant growth in purchases of both categories. This, of course, has had an impact on the value increase in e-books, with the average price paid for a nonfiction e-book some 74% higher than that paid for fiction in the first half of 2014.

The uptick in tablet ownership is also playing a role, enabling consumers to engage with illustrated and complex digital content. And, as tablet use rises, the number of e-books bought to read on dedicated e-readers continues to decline. Although there has been much speculation about when smartphones might begin having a significant impact on the e-book market, in the first half of 2014 just 6% of e-books bought by U.K. consumers were bought to read primarily on a mobile phone, the same proportion as in the first half of 2013.

With consumers now paying more for e-books, the actual purchasing decision is no longer just about price. Instead, it’s increasingly about the author or the subject—particularly important for nonfiction e-books, the category that has seen the most significant growth. But the survey shows that a slightly lower proportion of e-books were bought on impulse in the first half of 2014 compared to 2013, a phenomenon that may be linked to higher prices.

Library e-book lending in the U.K., meanwhile, has yet to come of age. Only a very small proportion of the 50,000 respondents interviewed for the U.K. survey said they had borrowed an e-book, whether from Amazon or a public library, or via an e-book subscription service.

Jo Henry is v-p of research and analytics, Nielsen Book. Jim Milliot is PW’s editorial director. More information on Nielsen data regarding the U.K. market is available from Hazel Kenyon (hazel.kenyon@nielsen.com). More information on the U.S. market can be obtained by contacting Charles Friscia (charles.friscia@nielsen.com).