I just tried to drag an audiobook into my iTunes queue, and it wouldn't let me. I then tried to add it to play next via the iOS Remote, and I couldn't there either, so it must be a lack of function rather than a UI oversight.
I understand that the two are different types of media, despite being transmitted through the same means. However, is it not feasible that I may want to chill to some music before delving into an audiobook, and then may want some music afterwards to relax themselves out of the tension of some exciting roller coaster of fiction. Better still, if audiobook publishers used .m4b files properly and added chapter markers, would it not be a neat experience to listen to an audiobook, with chapters separated by some tracks of your choosing?
One final note, I spotted in the iOS Remote app that the "Rewind 30s" button is displayed when an audiobook was playing, which is useful. However, I can't find such a button on iTunes itself.
(One last note - when viewing this post the i in iTunes looks uppercase. It is not; I wrote the i in lowercase, it just seems to display merged)