Although a long-time pseudo-feature of Amazon's Kindle hardware, its iOS app of the same name now offers the ability to read your books to you with Siri's voice. The downside? You can't just have Siri read your books to you with the tap of a button.
Amazon added this feature to support accessibility for the blind. You have to enable VoiceOver in the accessibility settings in iOS before you can have Siri read to you, and even then she only takes one line at a time as you move your finger down the page. It's not the sort of thing you'll actually want to use if you can read with your eyes. For those who are blind or have poor vision, VoiceOver support makes navigating the Kindle app and reading books possible.
Amazon added a variety of other new features as well, including support for its X-Ray service, which provides supplemental behind-the-scenes information for supported Kindle content. It has also added highlights, bookmarks and notes. For more information on all the new features, check out the full press release over at Amazon.