Romance fans browsing Amazon for their latest steamy read are increasingly being offered virtual door-stoppers at bargain basement prices. But some authors say these self-published ebooks, which run for thousands of pages, are a money-making scam designed to exploit Amazon’s algorithms.

It works like this. Amazon’s Kindle Unlimited service, which gives people unrestricted access to a selection of books for a monthly fee, pays authors based on the number of pages read, up to a maximum of 3000. To reach that maximum, some authors …