Realistic Drawing - Shading Practice

After looking at some feedback from peers and reflecting on my past drawings, I had decided to work on my shading techniques for drawing 3D objects. In the sketch below, I focused on using some basic 3D shapes as they are easy to practice on. In the book I am using, The Big Book of Realistic Drawing Secrets, the author gives many bits of advice when drawing shadows of objects, such as "Angular objects will have a more abrupt value change" and "Round objects will have gradual changes". There was a lot of tips, but to summarize, the author went into detail about how certain objects will be affected by certain light and where the source is. The book also stated to identify a source of light to find the natural shadows, which did help me make the shadows direction consistent. One final thing about shading was how "smudging", which is a technique where your use your finger to pull the lead on the paper and spread it, making it fade out, isn't a good technique and shouldn't be used.

Using the examples and tips in the book, I drew a rectangular prism and a sphere to show the different ways the shadows form on the two shapes. I chose not to smudge, since the author didn't advise it as being any good. I also made it so the shadows are coming from a source of light to the left and slightly above the objects

Overall, I think practicing this has helped me understand how to draw shadows consistently, and that learning this now will help me with future drawings. Is there any advice you would like to give me or any areas of improvement where I should focus on next?

Parks, Carrie Stuart., and Rick Parks. The Big Book of Realistic Drawing Secrets. Cincinnati, OH: North Light, 2009. Print.