He could take the idea a lot further. For example, he staggers the work of the writer and the editor. This is "mini-waterfall". Sure, it's better than one big bang of editing at the end, but it's still not doing the work simultaneously. Or writing one chapter per iteration. That only works if you can publish each chapter on its own. Each iteration should end with a potentially shippable result. It would be better to write a prose-style outline of the whole book, with maybe a little bit more detail in some key part.

Those observations aside, it is nice to see someone thinking about applying agile outside of software.