Learning Theme Coding

Hello,

I am a web developer who does quite a bit of WordPress work. I want to learn as much as I can about theme development. I would really appreciate some links to websites, or better yet, books that explain theme development in depth, I understand intermediately how to develop themes (I watched the Lynda.com video), but I want to learn about more advanced functionality so I can really start to create highly effective websites in WordPress. I already know the value of the Codex.