In its annual summary of ISBNs registered for self-published works, Bowker reported that nearly 730,000 were issued in 2015, up from 153,000 in 2010. The numbers cover ISBNs issued for both print and digital formats.

The ISBN is a useful way to monitor sales across the supply chain, but works published on a single platform can forgo the identifier and rely on platforms such as Amazon to report performance. Because the creators of many self-published works do not apply for ISBNs, the number of new works published each year is believed to be greater than Bowker is able to report.

When the number of self-published works was lower, this gap in the data was something the publishing community could live with. As the number of self-published titles has grown, however, there’s a sense that the share of sales for these works is also growing.

The debate about the true size of the self-published market continues. Traditional publishers and advocates for independent works have each tried to position the market as more or less tilting their way. To date, we’ve not seen much of a discussion about why knowing the size of the book market matters.

Here’s one reason it matters: mobile content discovery and consumption.

Pew Research’s recently published study “Book Reading 2016” has been widely cited within industry circles for its finding that 28% of the U.S. adults it surveyed had read an e-book in the prior year. That number has held steady since 2014.

Traditional publishers, perhaps tired of the constant stream of digital disruption stories, welcome these numbers as a sign of stability. With 65% of those surveyed claiming they read a book in print form, print remains the dominant format for reading.

What’s missing from that analysis, though, is how physical and digital books are bought. When Bookstats, a joint effort of the Association of American Publishers and Book Industry Study Group, was disbanded by the organizations themselves after reporting on 2013 sales, online sales of physical and digital formats totaled more than 51% of sales at retail. The share of books purchased online had grown steadily from 2008 through 2013.

Even in 2013, it was clear the tipping point had been passed. Online discovery and purchase was the single largest retail channel at that time, and its lead has likely continued to grow.

The movement online is increasingly intertwined with mobile devices. Five years ago, tablets and cell phones lagged behind dedicated e-book readers as the device of choice for consuming e-books. In 2016, consumers told Pew that they are now most likely to use tablets and cell phones to read e-books.

This shift from dedicated e-readers to multifunction devices offers important opportunities to companies that have been working to develop consumer-facing Web presences, such as HarperCollins. Online access and mobile connectivity provide a chance to develop true first-party relationships, outside of those mediated by online retailers.

Those first-party relationships can help publishers personalize offers, identify readers across multiple platforms, and broaden the ways that they reach existing and new audiences. Publishers can again become allies with readers in a market where content abundance is a challenge.

As multifunction tablets and cell phones come to dominate the e-reading landscape, they also pose challenges. The threat of substitute products is higher, particularly on devices that are already known for information “snacking” (or consuming short-form content). The presentation of traditional book content, already challenged on the smaller tablet screens, increasingly must be rethought for still-smaller devices.

Taking advantage of these new opportunities requires investments. That’s why the size, shape, and trends of the online marketplace matter. It’s not just a debate about traditional versus independent publishing, although that discussion will go on for some time. Understanding the market gives authors and publishers the data needed to inform where and how they spend their time and resources.

Using mobile access as a gateway to establish direct relationships is not easy work. But the data we already have show clearly that online purchases and mobile access are overtaking bricks-and-mortar channels.

This isn’t an argument against sales of physical books in physical bookstores. That channel will continue to serve book publishers well. But the growth in online purchases and consumption gives the publishing community a chance to address another Pew trend: a steady decline in the share of U.S. adults who say they read even one book in the past year. That’s where consistent use of identifiers can generate better sales data, which in turn lets us understand and serve the market.

Brian F. O’Leary is founder and principal of Magellan Media, a management-consulting firm that works with publishers.