Why do my audiobooks keep disappearing off my iPod

For the past couple of years every time I download an audio-book from Audible I have to authorize my computer all over again & authorize the audio-book all over again. Two weeks ago I bought new music CDs and added them to my iTunes library, then synced them to my iPod Touch in a playlist which as the time has gone by I've modified to tweak the list to improve it. Tonight when I synced my list all of my audio-books were removed from my iPod. This has happened once before & I had to re-download all of my audio-books from Audible & sync them all over again. Why does this keep happening & it always seems to happen shortly after a new update to iTunes. (Not that there isn't always a new update to iTunes) I paid good money for these books & I like to listen to them on my iPod. Why does iTunes not seem to be able to save my authorizations to play these files? This situation of having to authorize my PC for my Audible account & iTunes has been going on for roughly 2 years. Why hasn't this thing been fixed?