Earlier quoted context omitted.
Have you tried VoiceOver? On iOS, turn it on by triple-clicking the Home button, then two-fingered swipe downward to start speaking continuously until the end of the book.
Well that's intuitive...
However, Apple and Amazon's hands may be tied by contracts. Amazon tried to introduce general-purpose text-to-speech in the Kindle 2, but soon retracted it when rights-holders asserted that such a feature required paying a separate audiobook royalty.
Roy Blount Jr., in his role as the president of the Authors Guild, set forth that position in an NYT editorial titled "The Kindle Swindle". Noting the public opposition of the National Federation of the Blind, he specifically exempted assistive technologies.
http://www.nytimes.com/2009/02/25/opinion/25blount.html?_r=0