“A not-insubstantial chunk of their infrastructure is based on their reviews — and all of that depends on having reviews customers can trust,” said Edward W. Robertson, a science fiction novelist who has watched the debate closely.

Nowhere are reviews more crucial than with books, an industry in which Amazon captures nearly a third of every dollar spent. It values reviews more than other online booksellers like Apple or Barnes & Noble, featuring them prominently and using them to help decide which books to acquire for its own imprints by its relatively new publishing arm.

So writers have naturally been vying to get more, and better, notices. Several mystery writers, including R. J. Ellory, Stephen Leather and John Locke, have recently confessed to various forms of manipulation under the general category of “sock puppets,” or online identities used to deceive. That resulted in a widely circulated petition by a loose coalition of writers under the banner, “No Sock Puppets Here Please,” asking people to “vote for book reviews you can trust.”

In explaining its purge of reviews, Amazon has told some writers that “we do not allow reviews on behalf of a person or company with a financial interest in the product or a directly competing product. This includes authors.” But writers say that rule is not applied consistently.

In some cases, the ax fell on those with a direct relationship with the author.

“My sister’s and best friend’s reviews were removed from my books,” the author M. E. Franco said in a blog comment. “They happen to be two of my biggest fans.” Another writer, Valerie X. Armstrong, said her son’s five-star review of her book, “The Survival of the Fattest,” was removed. He immediately tried to put it back “and it wouldn’t take,” she wrote.

In other cases, though, the relationship was more tenuous. Michelle Gagnon lost three reviews on her young adult novel “Don’t Turn Around.” She said she did not know two of the reviewers, while the third was a longtime fan of her work. “How does Amazon know we know each other?” she said. “That’s where I started to get creeped out.”