Gnome javascript documentation
To start writing GNOME Shell Extensions, I would recommend you to start looking the Shell Extensions wiki page. There you will find links to tutorials, as well as guidelines.
With respect to modules available, you have everything that is available via GObject Instrospection. In order to get familiar with this, you can take a look the explained demos. For API docuementation, you can go to Platform Overview.
Not an expert myself, but based on my experience patching an extension, most of the utility are based on Clutter in which GObject Introspection expose the API to JavaScript. Unfortunately I cannot find the API documentation for the shell itself. The closest I can get is the JavaScript for the shell at /usr/share/gnome-shell/js/ which (for me) is quite enough as an extension would probably modify JavaScript object at runtime rather than calling it's function.