But there are several factors that might have made book sales at the beginning of this year slightly worse than those in the same period last year. Like the movie business, publishing depends heavily on a few outsize hits each season to drive profits. In the early part of this year, there wasn’t a huge, breakout best-seller, certainly nothing like 2015’s “The Girl on the Train,” which came out in January and sold two million copies in just over four months.

The adult coloring-book fad, which provided a huge boost to publishers and booksellers last year, has started to fizzle, possibly driving down sales this year. (In 2015, some 12 million coloring books were sold in the United States, up from one million in 2014.) And the surge in downloadable audiobook sales might account in part for the decline in hardcover and e-books, if more people are listening to books instead of reading them.

But perhaps the biggest factor affecting publishers’ revenue, and one that is not likely to go away soon, is the decline in e-book sales, Mr. Cader said. While publishers once fretted that digital book sales were eroding more profitable categories like hardcover, they now are finding that e-books — which cost next to nothing to produce and zero to ship and which can’t be returned as unsold merchandise by retailers — are critical profit engines. But e-book sales have fallen precipitously for months, in part because many publishers have raised their prices after negotiating with Amazon and gaining the ability to set their own prices.

The decline of digital sales and stabilization of print may have also led to higher returns of unsold merchandise from booksellers, reducing revenue. And while some book buyers may have traded e-books for print books, others may be buying cheaper, self-published e-books on Amazon.

Those reasons may partly explain why separate data from Nielsen, which tracks only print trade book sales, looks so different from the publishers’ numbers. Nielsen BookScan data showed that sales of print books were up by 16.4 million units in the first half of this year, as Mr. Cader noted on Publishers Marketplace.