The iBooks app is available for iOS (the iPhone, iPod touch and iPad) and the latest version of OS X, Mavericks. It works very well on the iOS devices, but less well with speech on OS X.
The first time you connect an iPad, iPod touch or iPhone to the App Store you are asked if you want to download iBooks, the eBook app.
As well as working with the built in accessibility features - Zoom, Invert Colors and VoiceOver - iBooks allows you to change the font of book content up to approximately 40 pt and has three colour themes.
We've recorded some videos demonstrating the accessibility features of iBooks. We used an iPad for our demo, but the same feautres are available on the iPhone and iPod touch.
iBooks is available for Mac computers running OS X version 10.9, also known as Mavericks.
It looks and works much the same as the iOS version visually, but is not so smooth with VoiceOver. For example, some of the buttons are unlabelled. If you can work around issues like these, you can use it to buy and read books with speech or braille.