I think one thing that would make understanding hooks much easier is actually seeing the API2 calls used by the x3 interface. It's very difficult to understand hooks without having a basic knowledge of API2. I did a brief overview of API2 in my Customizing cPanel presentation at last year's conference which you can view at:
http://www.cpanel.net/docs/seminar07...ing_cPanel.pdf
You can find the source for the X3 interface in /usr/local/cpanel/base/frontend/x3 - looking at the actual API calls will help you learn what parameters to expect.
For a sample hook, look at:
/usr/local/cpanel/hooks/passwd/change_password.example
The accompanying page for changing passwords in the X3 interface is ...:2082/frontend/x3/passwd/index.html, which is stored in /usr/local/cpanel/base/frontend/x3/passwd/index.html
This should help shed some insight onto hooks and how they work, especially in conjunction with the actual API2 calls that trigger the hooks.