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

From WP:
Write a Book Support —

And a great example of WP book online:

What else is there?