I recommend audio books, for instance from iTunes Music Store.
They are great for use in the iPod when driving, or when walking, makes the constitutional also be educational.
Some of my favorites are the books by Douglas Adams, ready by himself. And the books of Steve Martin, read by himself. And Robin Williams, interviewing various creative/famous people.
One comment though: It seems that many suppliers of audio books, as well as e-books, are pricing them the same as printed books. This is just wrong on so many levels. Production and distribution costs are a tiny fraction of those of the paper counterparts, and to not pass on part of those savings to your customer is just greedy, and it is disrespectful of your customer.
Personally I would say that about half the price (at most) would be right.
Apple iTunes charges two dollars for a downloadable 40-minute TV show without ads. That is a good price. And they charge eight dollars for a 30-minute audio interview. That is not a good price.