...For much of the last year, mainstream novelists were furious that Amazon was discouraging the sale of some titles in its confrontation with the publisher Hachette over e-books. Now self-published writers, who owe much of their audience to the retailer’s publishing platform, are unhappy. One problem is too much competition. But a new complaint is about Kindle Unlimited, a new Amazon subscription service that offers access to 700,000 books — both self-published and traditionally published — for $9.99 a month. It may bring in readers, but the writers say they earn less. And in interviews and online forums, they have...