Agree, this is great news. I enjoy listening to audiobooks when I run and can now do so without carrying my phone. I tried the app out yesterday and seems pretty well done. I like how it shows the progress in transferring the audiobook to the watch, something you don't get when loading podcasts. I find it frustrating that to load podcasts you have to have it on the charger and it just seems to load whenever it feels like it, with no progress visible.
Only issue I have, and maybe I just haven't set it up right, is that that my progress in the book isn't retained when I transfer to the Apple Watch. For example, last night I transferred a book that I'm nearly done with on the iPhone to the Apple Watch. When I play it on the watch, it starts from the beginning.
I'll look into further to see if there's some sync setting that I'm missing.