Welcome to Discussions,
I have recently signed up to audible.co.uk to download their spoken word books. You have to download an additional element into iTunes in order to get the books onto iTunes and then onto an iPod.
If your profile is correct we both are using Mac OS X 10.4.11
I also have recently joined Audible and *did not need to download any additional software to add audibles books to iTunes*. The additional software is for Windows or Mac's running pre Mac OS X 10.2.8. Audible do recommend using the latest version of iTunes.
*It is very important to note that Mac users running Mac OS X v10.2.8 or later do not need to configure their Mac to use Audible. The Mac operating system is already fully set up to use Audible content. Software on our website such as Audible Manager, and Audible Download Manager for iTunes are only required for Windows users*.
[Configuring Audible for Mac Users|http://audible-uk.custhelp.com/cgi-bin/audible
uk.cfg/php/enduser/std_adp.php?p_faqid=3431&p_created=1164193168&p_sid=WpQioFi&p_accessibility=0&p_redirect=&p_lva=&p_sp=cF9zcmNoPSZwX3NvcnRfYnk9JnBfZ3JpZHNv cnQ9JnBfcm93X2NudD05OCZwX3Byb2RzPSZwX2NhdHM9JnBfcHY9JnBfY3Y9JnBfc2VhcmNoX3R5cGU9 YW5zd2Vycy5zZWFyY2hfbmwmcF9wYWdlPTE&p_li=&p
These additional elements that you have downloaded could be the cause of the error that you are experiencing.
Hope this helps,