In my classes I also recommend the book Made in the Shade, by CZT, Cris Letourneau. This is a great workbook that is full of wonderful information on everything to do with shading zentangles. Cris covers in depth, getting to know your tools, shading simple shapes, shading tangles, shading ZIA, and shares some new tangle patterns. The workbook is set up for you to practice what you learn every step of the way with examples to fill in right in the book. At the end there are a lot of samples for you to practice shading from a lot of CZT's and artists. So if you want to get more in depth practice with shading check out Cris Letourneau's book at this link to her blog.
As Rick & Maria from Zentangle Inc. say, "Not everything in life is black and white, but all shades in between".
(Note: If you made it to the bottom of this post and know how to show pictures side by side in Blogger, let me know. They are side by side when I am writing my post, but not when it is published???? Thanks)