Where does one learn to use GTK ?

I see many people praising GTK apps' looks and thight integration with many DEs. It then seems logical to want to learn to use it. The problem is, the official docs are great... For C, and other languages' bindings are not as good and sometimes outdated.

So this is my question, how does one learn to use GTK ? Looking at source code is great, but isn't there another way ?