I've just purchased an audiobook from the iTunes Store for my son.
Sadly, iTunes will not allow me to transfer the book to his iPod. It gives a warning message saying it could could not transfer the book as I am not authorised to do so on this computer.
I've never had a problem putting music on his iPod, and I have just bought this book from Apple for him and have not transferred it to any other device.
Best Answer
iTunes stores computer authorization as a secure certificate and sometimes isn't helpful in correcting problems with the chain of trust.
The general procedure to correct iTunes authorization issues is to perform the following steps:
At this point, the computer should have retrieved a valid authorization cert/token and you can try again to sync purchased content from iTunes to the devices it controls. Should my brief set of steps fail, there are many more resources on troubleshooting authorization issues here: