Not sure if it's too late yet or not, but I'd be interested in helping..
Not with SFML knowledge itself, but with the writing, in the sense that I'd like to review and change the book to make it much more user friendly. Groogy shared a draft of his book, which I corrected and modified to, in my opinion, make it much more so.
So yeah, it's up to him, I guess, since he appears to me like the "lead writer" of the project.
I'll just mention beforehand that I AM 14 years old. I honestly don't see it as a problem, but so many people lose their interest when they learn that, I'd rather specify in advance. It also means that I don't want any money whatsoever, I'm not interested by it, nor is it legal for me to be paid.
I'm also part of the IB program; the International Baccalaureate; in English, so I am taking literary English courses. I guess that means that I'm not stuck with basic grammar in school. :p
And finally, I'm considered the "grammar Nazi" of BoxBox's #sfml.
If not, I'm still excited to see this book in action!