Role cannot create or edit book content
I'm not quite sure why this started working, but all went right with the world when I added the "Book page: Edit own content" and "Book page: Edit any content" permissions for the "book author" role. 

Two odd things with this though:


I'm sure I tried this at least once before earlier today
I immediately disabled both permissions again to see what would happen, and nothing did, i.e. the book author role is still able to create and edit draft Book page content... which is good, but weird