The key phrase in question is any book or other work you generate using this software, defined as a “Work”. The “Work” in question is the generated output of the software, not the input.
In other words, Apple does not claim ownership over your writing, only over what you get back after you put it into iBook Author.
They did this with the App Store as well, and people are still complaining about it, but no one’s calling them fascists anymore.
Why? It worked.