The license is also stated on the index page https://github.com/rpicard/explore-flask/blob/master/source/... (live page: http://exploreflask.com/). He talks a little about this reasoning on that page:

> I finally released the book, after spending almost a year working on it. Almost immediately I was tired of managing distribution and limiting the book's audience by putting it behind a paywall. I didn't write a book to run a business, I wrote it to put some helpful content out there and help grow the Flask community.

> […]

> In the spirit of open source software, I'm placing all of the content in this book in the public domain.