Readers of this site will know that I have a very high regard for the tutorials produced by George Jardine. His series discussing the Lightroom Library and Develop modules are already linked from early posts.
Recently while I was in the US George dropped me an email to let me know that he had updated his tutorial describing how the Highlight and Shadow controls in Lightroom 4 and Camera Raw 7 work, and would shortly be publishing a video tutorial. The tutorial has been available for a while now, but the video has just gone live. You can read the tutorial at Revisiting the Zone System. Details of how to access the video tutorial can be found at the bottom of George’s blog post entitled ‘A Few Thoughts on Highlights, Shadows, Whites and Blacks‘.