Text-to-speech on e-books is going to be available only if the publisher has made the rights available. Most e-book readers will have the option available, but many e-books come with text-to-speech disabled. Check the fine print! (don't you love DRM?)

A quick and unscientific poll of the top NYT bestsellers on Amazon showed 3 out of 4 with text-to-speech enabled for the Kindle edition (Crusie, Patterson, Grisham) and one without (King).