Some established artists, however, see fewer rewards. Taylor Swift pulled her music off Spotify this fall, saying it was devaluing her art and costing her money. “Valuable things should be paid for,” she explained.

Holly Ward, who writes romances under the name H.M. Ward, has much the same complaint about Kindle Unlimited. After two months in the program, she said, her income dropped 75 percent. “I couldn’t wait and watch things plummet further,” she said on a Kindle discussion board. She immediately left the program. Kindle Unlimited is not mandatory, but writers fear that if they do not participate, their books will not be promoted.

Ms. Ward, 37, started self-publishing in 2011 with “Demon Kissed,” a paranormal tale for teenagers, and quickly became one of Amazon’s breakout successes, selling more than six million books, according to her website. She said in an interview that she does not understand what her partner Amazon is thinking.

“Your rabid romance reader who was buying $100 worth of books a week and funneling $5,200 into Amazon per year is now generating less than $120 a year,” she said. “The revenue is just lost. That doesn’t work well for Amazon or the writers.”

Amazon, though, may be willing to forgo some income in the short term to create a service that draws readers in and encourages them to buy other items. The books, in that sense, are loss leaders, although the writers take the loss, not Amazon.

An Amazon spokesman declined to answer questions about Kindle Unlimited. While Jeff Bezos, the company’s chief executive, celebrated “authors as customers” as recently as his 2013 letter to shareholders, and the retailer tried to enlist independent writers in its campaign against Hachette this summer, some self-published authors are beginning to suspect that they are just another supplier.

“Does Amazon want to become a legacy publisher like we all are fleeing from and they seem to disapprove of?” the horror writer Kathryn Meyer Griffith asked in an online forum, adding, “They’re doing a good job of recreating that whole unfair bogus system where they make the money and we authors survive on the pennies that are left.”