free doesn't mean that you don't have to pay ! Of course, it's always better, for the "client" when it's free of pay, but the GPL allow this.
For a such great tool, of course, you can understand a lot of things alone and the forum is really helpfull, but if you want to help them to continue like this, you can also buy this manual !
Free is wonderful
edit : (oups, it was a answer to the first page ^^)