The Audio Book Publishing industry has had one of the more fascinating histories over the past decades, compared to other industry segments.
While it is true that technically speaking, the concept and execution of recording spoken word read from a book has been around since a third of the way through the 20th century, it's really the past 10 or so years that things have gotten interesting.
And by interesting, I mean widespread; sonically excellent; transportable, lucrative; etc.
Consider the millions of iPods and iPads on the market, each able to retrieve and play audio book content. Millions. And they are not the only devices on the market, as Sony, Barnes and Noble, Amazon, and others have all created playback devices that will support the audio book format.
So when publishing your book, keep a keen eye on the audio rights, taking care to do your best to keep them. You'll notice that top publishers covet these rights, and for good reason. And therein lies the Zen message of how important and lucrative these rights can be.