Publishers Weekly has reported that industry revenue—i.e., revenue of the 1,209 publishers who report to the Association of American Publishers—rose 4.9% in 2014. It does not say whether the increase was due to greater unit sales or higher prices. Nor does the article report unit volume.
It does report that total adult book sales revenue fell 1.4%. On the other hand, within the adult trade segment, e-book sales rose 1% and accounted for 27.2% of adult trade sales (up from 26.6% in 2013). Trade paperback sales increased 2.5%; mass market paperback sales fell 4.2%, and hardcover adult trade book sales fell 8.2%.
An earlier story on the Publishing Technology website gave a little more detail about 2014's book and e-book unit sales. It pointed out that, based on Nielsen Bookscan data, e-book sales cycles are different from print book sales cycles. Print book sales tend to peak in the last quarter of the year as people buy books for holiday gifts; e-book sales have tended to peak in the first two quarters of the year as people download titles into the readers they received for Christmas.
The analysis suggested that e-book sales are not plateauing but adult fiction is declining—reinforcing the PW story. According to the Bookscan figures for all books, adult fiction accounted for 45% of the 650 million books sold in 2004. By 2014, adult fiction took a smaller slice of a smaller pie: 40% of 635 million books. In other words, if these figures are accurate, the industry sold roughly 39 million fewer adult fiction titles last year than it sold in 2004. Sobering news for anyone who writes adult fiction.
One comment to the Publishing Technology story points out that these figures are based on ISBN numbers and "since the majority of e-books published by independent authors rarely have ISBN number" they are not counted in the figures. It suggests to this writer that "more and more readers of e-books are switching to independent authors, and traditionally published e-books are paying the price." That would be good news for adult fiction authors if true, but without a reliable source of numbers there is no way to know.