I first learned about volatility in Hull's textbook, which you should already know.

Then I was introduced to the practical aspect of volatility in Natenberg's "The Option Trader's Guide To Probability, Volatility And Timing".

For academic side of things I know these 2 books can be downloaded here, but I haven't read them bcause I'm more focused on actual trading:
"A Practical Guide to Forecasting Financial Market Volatility"
"Forecasting Volatility in the Financial Markets"

2# ectopic
cheers, man. thanks for your advice. i really got a lot of help from your words. another problem is, do you think it would require a advance math skills for finish the course? i mean, for example, stochastc skills or something like this. thank you very much. have a good night~

Good ideas!

not quite necessary, for basic knowledge
learn some basic math and statistics
3# hantb4510