I've been playing around with Audio Books and my Creative Nomad Zen Xtra a LOT. Yes, it is easy to rip Audio Book CD's and get excellent results. The Zen Xtra is great with Audio Books. It remembers where you left off, plus it has a bookmark function that's handy if your chapters are long.
For MP3 playback, Audio Books on CD's are preferable to Audio Books on Tape, for several reasons. One, you can rip a CD much faster. Two, chapters are usually between 3 and 6 minutes on CD. They do this, because some CD players do not remember where they are in the middle of the track, but will remember what track you are on. I have a MP3 CD player that behaves this way. For example, if you make one track 45 minutes long (length of a typical tape). These types of players will go back to the beginning of the track when you turn the power off. If you're 40 minutes into the track, that's not good. With CD's, you only repeat 3-6 minutes, which is not bad.
But again, the Zen Xtra does not have this problem since it remembers where you left off, plus you can set a bookmark.
I have found an issue with Media Center and certain low MP3 bit rates. I am currently trying to track the problem down with Creative. So far the issue appears to be with MC MP3 encoder. I have found that MP3 bit rates recorded at 32kbps do not play well on the Zen. That is why I have standardized on 64kbps for Audio Books. Although 40kbps seems to work fine as well. I have not experimented with WMA.
For your information, there are plenty of MP3 Audio Books that are recorded at 8kbps and 16kbps for free. See
www.AudioBooksForFree.com . These files do not play well on the Zen Xtra. I find 8kbps to be terrible sound quality, but 16kbps to be tolerable. But there seems to be a firmware bug on the Zen Xtra which makes 8kbps and 16kbps files unlistenable regardless. For free, I can tolerate 16kbps on devices that play them properly. I do note that MC does not allow you to encode MP3 below 32kbps, nor does it have an option to encode mono. Both would be nice options for recording of Audio Books.
Regarding Audible.com. Creative is working on support for aa files. Neither site claims support yet. But I did get a tech support person say that the companies were working together. It will probably be a future firmware upgrade for the Zen.
Chris