Gartner and other analyst firms estimate that Amazon's Kindles make up about half of the market for black-and-white e-readers, not including devices like or the . In addition to its Kindles, Amazon supports other platforms for reading its e-books, including Android-based devices, the , the and the BlackBerry, as well PCs and Macs.

James McQuivey, an analyst at Forrester, said that Kindle for the Web "is all the more reason to say that cloud-based reading is not a strong differentiator for Google on the consumer side."

With Amazon's ability to let independent booksellers sell books through Kindle for the Web, as well as Google's similar capability, McQuivey questioned whether an independent seller would be "happier to feather Amazon's nest or Google's nest? Which enemy do you want to strengthen?"

Those independent sellers will face that tough decision between Amazon or Google "if they want to be in business five years from now," McQuivey added.

While Kindles hold half of the black-and-white e-reader market, the more serious question is how many books Amazon has sold through its e-readers. McQuivey said Amazon has about two-thirds of the e-book market. "I don't expect that to really change," he said, even with Google's announcement.