Actually resume stuffing was the major reason (not to get a job but rather for positioning oneself). I wrote it in my free time actually (my income on the book is about a 10th of the time put into it). That it is free to the students is just a nice side effect. The target audience is decision makers in smaller companies. You do not reach them via wikibooks. I guess I would have never written it as open access, because in this case I would have left it to others.

I would like to see a peer review/editing community for wikibooks btw, that actually would lead to better quality of books. distill.pub eg IMHO is great.