In the U.S., business websites are required to be ADA-accessible. It's conceivable that Amazon is adding this to eBooks now, though generally eBooks by their nature are pretty accessible. Images would require special attention. I doubt very many of us use images, but if you do, they should have ALT text in their tags so readers for the visually impaired would be able to describe the image for those that can't see it. Also, and again most of us don't do this, but if you are putting text in specific colors or using backgrounds and such, you want to make sure there is a high enough contrast so it is readable by people with poor vision. And so on.
Of course, that's a guess but that's what comes to mind when someone says "accessibility features."