Drop Down List again

Sorry that I post this discussion again but I've see that it is private and I want it public.

I have a module and for a menu link will be displayed a form.
1. In this form I have 2 select element. First element is populated with some authors name. The second select element must be populated with the books for the selected author from the first select element.
2. For each selection in first select element, the option array for the second element must change.
3. I must have a second submit button, or a button and have option to call a function when I press on it.

If anybody could help me with any idea, suggestion, documentation.

Thank's!