I think some features were sponsored by some companies that use RT, but the bulk of the work is voluntary, as far as I know.
Which companies are using RT?
Well I have started this thread, but it is funny: I have done quite a bit of work without being payed for it as well, at that time at least. Gábor would say “bagoly mondja a verébnek…”
Concering books and their cover or contents: cover - no idea to be honest.
Contents - I have a blog where I am documenting my photographic work - there are also some tutorials for RT. Basically I am just making screenshots of my settings when I develop photos. I like to pass on my knowledge and I am thrilled about this program but I am not really a RT expert (yet, at least). The URL is in my pixls.us profile.