Bookmarks work for me with Video files. I have not tried Audiobooks.
I just read through some of the wiki on bookmarking and found this near the top:
"If the file in question has the [Use Bookmarking] field set explicitly to Yes the the Bookmark is obeyed. If it is set to No then it is ignored. Otherwise, if [Use Bookmarking] is set to Default, then it will automatically use Bookmarking:
If the [Media Type] of the file is Video and the duration of the file is greater than 15 minutes.
Or if the [Media Type] is Audio and the [Media Sub Type] is Podcast or Audiobook."
This explains why bookmarking has always "just worked" for me on video files, but not on songs (music) files. I wonder if your audio books have the proper Media SubType set? You might also try manually setting the [Use Bookmarking] field in a file or two to "Yes". I'll bet it's something to do with the media type/subtype.
Brian.