Apple just released iBooks2 and the accompanying iBooks Author is live and free on the App Store. Upon launch it has the feel of Pages and Keynote with a template screen to get you going right away.
After choosing a template you are given a pre filled content of a Chapter 1, Section 1, and Page 1 to guide you. In the fantastic standard of Apple, all elements are movable with X and Y coordinates as you drag.
But even more exciting is the iPad Preview button, that when clicked prompts you to select an iPad. This seems like a very simple way to sample your design.
Publishing is also built right into the software along with multiple ways to share work in progress by email, PDF and export as a text document.
Apple seems to have built a wonderful way to design not just text books but any type of interactive document with advanced HTML and rich media functions. Get it now in the Mac App Store for free.