I have found lots on going from a blog to printed book, but not so much on using WP to create an online book. Below are some links that I found helpful. Let's keep this thread moving forward.
A great discussion on the topic:
An Open, Webby, Book-Publishing Platform - O'Reilly Radar
Write a Book « Support — WordPress.com
And a great example of WP book online:
What else is there?