I just heard about tracking completed downloads on another forum and thought it sounded useful.
You couldn't point me in the direction of that thread could you? It would be handy to know about.
On the MySQL subject, it's pretty easy to pick up and difficult to master (especially with multiple joins). For most websites though you don't need to know more than a few things.
some books I can recommend for learning PHP/MySQL:
PHP Hacks
Essential PHP Security