I wish to develop WordPress styles and plug ins. What should i know to get this done?

I've understanding of PHP, HTML, CSS and JavaScript. I've created a couple of projects with such. What's the best starting point (except Codex), can there be any book? How can i understand how the core of WordPress works?

Read this neat infographic for any good summary of the anatomy of the wordpress theme.

http://yoast.com/wordpress-theme-anatomy/