There are two notable efforts to make available numerous copyright free public domain books in audio format. There is no copyright on these books so the material is free to own, use and distribute.

Project Gutenberg offers a wide selection computer generated audio books. Which means the book is turned into audio by computers and not read by humans. Though this method helps the conversion process, from text to audio, much faster, there are some anomalies.  Project Gutenberg is working at resolving these anomalies.

Another big source of free public domain audio books is librivox.org. Audio books at  librivox are human read. Volunteer read and record books for free distribution. All books are public domain and so no copyright issues. They have a huge selects.In fact their catalogu is so big that its a daunting task to navigate and find the book of your choice. All their audio books are available in multiple formats like MP3 and ogg format. They can be played on most mp3 players including ipod and also can be played on a computer. Entire audio books can be downloaded in a single zip file or can be subscribed in iTunes for ipods.