Hello!
In http://danielpocock.com/rtc-quick-start-becoming-a-book-now-in-beta I read: "Which is the best license to choose to engage contributors and translators and ensure the long term success of the project?"
My own recommendations are:
• GNU GPL v2 https://www.gnu.org/licenses/gpl-2.0.txt if you want a license with copyleft
• Expat/MIT http://www.jclark.com/xml/copying.txt if you prefer a simple permissive non-copyleft license
I hope this may help the copyright owner(s) for this book in choosing a good and widely used DFSG-free license.
Bye.