I tried the "Outline" button and I found an interesting feature, called Weight. By using Outline one can put blog posts into the book hierarchy. And by doing so you can set a "Weight" number for it, and the text sais: book pages are ordered first by their weight, second by their name.
It would be a great improvement if we could set the weight when directly posting in the book hierarchy. Because like this I will rather use my blog to write chapters and outlining into the book hierarchy.
Some other thing I noticed: the book pages cannot be deleted. At least I couldn't find any method for it. It's really annoying, because I really don't know when I start writing a book what hierarchy I will end up. It's really hard to change the hierarchy of chapters after posted in book format.
And another small problem wich causes no real problem: I can't really use the image uploader, but the file uploader is just as good for the purpose.