One month after buying a book from Amazon, readers (the first 100) are sent an email asking if they have any questions for the author. After the questions have been submitted, they are collected and sorted by the Amazon podcast team who then interviews the author (those that participate). All subsequent shoppers (> shopper No.100) now see a little podcast logo next to the book. Clicking on it plays the author interview. This gives subsequent shoppers (> shopper No.100) another way of evaluating a book (beyond customer reviews, which are great, by the way).
Think of this more as a The Daily Show author interview for all books.