Some producers of free e-textbooks have had trouble persuading professors to adopt them. So one backer of “open-source textbooks” has decided to sell its titles on Chegg, an online textbook retailer, for a small fee in hopes of reaching a wider audience.

The group is called the Twenty Million Minds Foundation, and its goal is to save students money by creating e-textbooks for popular subjects and making them available free—or as close to free as possible. It has spent about $1.5-million developing a handful of textbooks written by high-profile scholars.

But its leaders admit that professors have been slow to assign the books in their courses. Few professors have heard of the Twenty Million Minds Foundation or of OpenStax College, the Rice University-run service that hosts the free textbooks produced by the foundation.

“When you’re a foundation and you approach a professor, they say, Who are you guys? You always get this really interesting stare, Why would I use your book?” said Dean Florez, the organization’s president and a former California state senator. “We need more adoptions, so we need to be side by side with a site professors trust.”

The group talked with several online retailers, including CourseSmart and AcademicPub, before deciding to work with Chegg, said Mr. Florez. One reason for the choice was that students already associated the site with low-cost options. “This gives us the ability to then, if you will, kind of hyperpush these books out into the marketplace,” he said.

The books will not be free, though. Chegg will charge a small fee that Mr. Florez characterized as a “minuscule” amount of a few dollars per e-book. “It’s less than the sales tax that students pay on the books that they’re buying from publishers,” he said.

None of that money will flow back to the Twenty Million Minds Foundation. Instead, some will support Chegg’s online reader software, which adds extra features to the original content, and some will support the OpenStax College.

The foundation could build its own software platform with such services, but Mr. Florez argued that open-source publishers would be best served by being in the same stores as all the rest of the books on course syllabi.

At least one traditional publisher has also started letting professors mix free content with its own material. Pearson recently released what it calls Project Blue Sky, which supports the creation of customized books from the Internet and from Pearson.