What’s the license for the book?

[JanX2]

I would recommend something like “Creative Commons Non-Commercial Share Alike 3.0”.

http://creativecommons.org/

[albertodebortoli]

Thanks @JanX2 for the suggestion.
We went for the "Creative Commons Attribution-NonCommercial-ShareAlike 4.0".

[JanX2]

Awesome. Thank you!