Such steps help explain why data from the National Association of College Stores, a trade group for thousands of campus retailers, show that student spending on books and supplies generally has been flat or declining, even though textbook prices have risen. Average annual student spending on required “course materials” — a category that includes new and used textbooks, access codes and digital books — declined 14 percent to $602 for the 2015-16 academic year, from $701 in 2007-8. (Last year’s spending, however, was an uptick over the average of $563 in 2014-15).

The main reason students acquired an access code, the college store association’s research arm said, was that their instructor required it.

Richard Hershman, vice president of government relations at the association, said the new report “raises a number of valid concerns around digital.” He said that pricing and distribution models for digital materials were evolving and that student concerns should be taken into account. Faculty typically decide what materials are required, he said, and many instructors see online tools as helpful to students. It may be, he suggested, that they can offer an “opt out” alternative for students who are unable to purchase them.

Student advocates say they worry that the proliferation of digital access codes may make it harder for students to use cost-cutting alternatives, like sharing — or even skipping the textbook purchase entirely. The move to unique digital codes essentially rules out sharing, they said, since the codes are usually attached to an individual student account and, once activated, cannot be reused.

“For any student who was not paying full price before,” Mr. Senack said, “this is definitely a concern.”

Jeanne Ryder, a sophomore at Rutgers University, said she learned about the drawbacks of access codes last year, when she spent hundreds of dollars on a hardcover Italian textbook that was stolen, along with her backpack. The book had come with an online activation code, she said, but it was missing and the publisher told her she would have to buy a new one. She was unable to obtain a new replacement code, even though she had her receipt. She ended up borrowing the book from another student.

Here are some questions and answers on college textbooks and access codes:

How can I keep the price of textbooks manageable?

Most students visit their campus bookstore for convenience, but it’s also wise to check prices online. Sites like CampusBooks.com compare prices for renting and buying from various online merchants. Also, you can check with your school’s library to see if it makes copies available, or if it offers open source textbooks.