Back in the working days I often had all day drives where I would listen to audio books.
On our last "big trip" I got the DW talked into listening instead of reading, that way I got something to do besides "drive".
For sources, don't forget Project Gutenberg. Besides free e-books, they have free audio books, read by volunteers. Some of these are very good (OK, so the guy reading the first chapter of "Around the World in 80 Days" mispronounced the names so badly I never finished listening)
Project Gutenberg Audio BooksThere are also other sources of free (mostly classical) audio books.
One thing I did was use an application that zips the audio files into larger chunks (QuickTime has a limit of 12 hours). This allows me to load the MP3 player with one file per for most books.