I have not yet given it a try, but a new application for Mac OS X looks like it may be even better than Join Together 5, which I recommended back in July. Audiobook Builder appears to go quite a bit beyond what Join Together is able to do, including the ability merge individual CD tracks into chapters before merging the chapters into a chapter marked whole-book track, add artwork to each chapter, etc.
My initial suspicion, based solely on looking at the screenshots, is that it may be a little complicated if your needs are simple. For myself, I usually only want to merge entire CDs into chapters, which iTunes does if you use the %(itunes)Advanced > Join Tracks% command, and then join those into a whole book, with chapter marks and full audiobook format and metadata, which Join Together does quite ably, and very simply.
In any event, I’ll be giving Audiobook Builder a try in the near future, and I’ll post a full review then, but in the meantime, I wanted to point to it, and see what other people think.