OP here. Just came across it and I found it was interesting enough as there's not so much books about Docker.

There aren't a lot of books about Docker because by the time they get published, Docker has made so many breaking changes, they are irrelevant.

While you're definitely right when it comes to printed books, self-published books tend to be pretty up-to-date (as an example, I believe "Agile Web Development With Rails" runs a test suite against Rails and the author gets notified the moment something's not accurate with the book, maybe it's the case for this one).