Amazon’s ebook subscription service, Kindle Unlimited, has attracted criticism recently, with some self-published authors complaining that the service devalues their work and chafing at the requirement that they make their ebooks exclusive to Amazon in order to participate.

But Russ Grandinetti, Amazon’s VP of Kindle Content, suggested at the Digital Book World conference in New York on Wednesday that the vast majority of authors participating are satisfied with Kindle Unlimited — and he said that the program is helping them achieve earnings that have doubled since the program’s launch in July.

Authors who want their books to appear in Kindle Unlimited have to enroll in KDP Select, a program that requires them to make their ebooks exclusive to Amazon for three-month periods. “Every month authors have renewed availability of titles on KDP Select in excess of 95 percent before and after the launch of Kindle Unlimited,” Grandinetti said — suggesting that they…