This is a new reading app for iPhone and iPad that let you seamlessly switch between reading a book and listening to the audio book. Read a chapter before you go to bed, pick right up where you left off, on your way to work in the morning. (the audio books are narrated by professionals, this isn't text-to-speech quality)
Its brand new, so the titles are limited, but their website says they're adding new books all the time.
OR – You can do the same thing on the Kindle app --they call it Whispersync for Voice-- it switches between your Kindle book and the Audible audio book. (Audible requires a subscription)